AK-74/U (early) [Cordon] - This one is only listed because you can get it right after you get your first weapon. South of the beginning camp in Cordon'there's a series of Military patrols on the road in packs of three. You can fire on them and lead them back to the stalker camp to loot their corpses (SMG's). You'll likely be light on ammo unless you do this repeatedly, but if you hang on to it, it'll come in handy for some tight situations in the future.
Silenced Pistol [Cordon] - This one is debatable on whether it's worth picking up or not, but if you're a stealth player and like to sneak around and be a true loner, you can kill "wolf's men" off and one of them drops a silenced pistol, which is the first one you'll see in the game.
Home-made Stalker Suit [Cordon] - Nimble gives you a mission to find the perfected suit, but he's an ungrateful bastard and doesn't compensate you nearly enough for it. I recommend finding the armor, then using it, as it's the best armor you'll get until you reach the bar. Good armor makes all the difference in the world in this game, so trash those leather duds ASAP.
Suppressed AK74-U [Garbage] - After you help Baz clear the junk yard of bandits and do whatever quests he offers, smoke his ass and take his sweet sweet gun. This is a silenced SMG, the first one you'll find in the game.
Upgraded Handgun [Agprom Station] - I don't remember the name of this gun exactly, but it was sweet enough for me to pop Mole in the dome and take it from his corpse. After you rescue Mole from the military squad, he'll lead you to a tunnel where you proceed to the next level. Before you go, take him out and steal his gun.
Fast Shooting AK74-U [Agprom Station] - This is an awesome gun. I put a scope on it and used it until the X16 labs. Anyway, it's in Strelok's hideout in the tunnels. As you enter the room, it's on your right laying on a shelf.
Warsaw Scope [Dark Valley] - This scope only works on AK/74-U's as far as I know, but if you have the above mentioned fast shooter, it's definitely worth picking up. You can't mount it on the suppressed AK/74-U, however.. I tried : Anyway, as you enter the Dark Valley, you'll hear some people talking. Sneak up closer and you'll see that it's a bandit, and a guy from Duty. The Duty guy (Bullet) is negotiating a hostage release, and you're going to follow him. The bandit falls alsleep for whatever reason, so if you're feeling devious, put him to sleep permanently as you pass. When you get to the road, Bullet will crouch down behind a wall. Go to the opposite side of the street and get ready, you guys are going to ambush some bandits. The trick here is to wait until Bullet shoots first, or the mission doesn't work. After the bandits are dead and the hostage is safe, you'll get the warsaw scope from Bullet's buddy.
PSZ-9M "Duty Armor" [Dark Valley] - This is in the basement of one of the buildings near Barov. It's pretty close to where the Duty guy is locked up in prison, so if you see him on your map look around in that area. It's just sitting out on a table. It's literally the best armor you'll get in the game until you see the SEVA suit or PSZ-9Md much later.
Storming Okaban [Dark Valley] - This gun is meh.. If you like grenades, I'd recommend picking it up because it has a launcher attached to it. I prefer the fast shooter with a scope though. Anyway, this one is easy to get, just pick up the quest from the drunk Duty guy in the bar, and you'll get a little mini-map icon to follow around. The reward he gives you is pretty lame, so if you like the gun, by all means keep it.
Rocket Launcher [X18 Labs] - This one is in one of the lower rooms in the X18 lab. The room with the giant mutant that shakes the ground. One rocket, and it weighs a ton. I carried it to the basement and blew the fire demon away, then dumped it. Feel free to do the same.
IL 86 [X18 Labs] - You'll find this on one of the orange suit guys in the very bottom of X18 (in the room with the fire demon thing that spawns burner anomalies). It's kind of a crappy gun though, and you won't find much ammo for it until later areas, so feel free to pass on it.
Big Ben [Rostok] - This one is in an underground parking area near Krugev's hiding spot when you first enter the area. It's inside one of those little briefcase looking things inside an overturned dumpster. It's a big gun. Very big.. according to the description it was modified to fire rifle cartridges. I didn't really use it, but it's worth a fair bit if you need money.
Black Kite [Rostok] - Literally 2 feet away from Big Ben is a corpse, and he's holding the Black Kite. Again, big gun, not really all that useful. Sell it or leave it imo.
Pellicle (Artifact) [Rostok] - In the same underground parking area you found those last two guns (and the 'family rifle'), there's this little guy. He's hiding behind a white car, and you can only see it from certain angles. It looks like a glowing green/blue blanket lying on the ground by the car's tire. It gives you 30% Chemical Burn protection with no side effects. Very very rare artifact, as far as I've seen this is the only place you can find it in the game (and I've played through from start to finish 3 times).
Lightened IL 86 [Warehouses] - Kill the stalker named "Master" in the little camp across the road from the army warehouses. This gun is lighter than a normal IL 86 (3kg vs 5kg), is less accurate, but has a silencer.
Balanced IL 86 [Warehouses] - Kill the stalker named "Smartass" in the little camp across the road from the army warehouses. 'Balanced' for more stability. This upgrade makes the IL 86 usable, if you're still using a fast shooting AK/74-U, you might want to upgrade to this. It's still not a very good gun though, but it seems to jam less than others of the same class.
Upgraded Shotgun [Warehouses] - There's another stalker in that same camp that has this gun. I don't remember his name, because I've literally killed 30 or so stalkers in that camp, and his corpse is underneath a mountain of bodies along with Smartass and Master. Anyway, from the item's description, this shotgun has been fitted with a small fragment of the Gravi artifact to make the shots fly faster. I'd assume this increases your impact rating for this gun, but I never tested it out.. not much of a shotgun user.
Vintar BC [Army Warehouses] - This sniper/smg is awesome. Extremely accurate and powerful, it's also completely silent. I took this gun and assassinated Skinflint in the freedom base and the guy guarding the armory down the hall without alerting a single Freedom soldier. The only downside is that it only holds 10 shots per clip, so you'll be reloading often if you use it in close quarters. It's in the building to the far north-east in the Freedom base, in the north-east corner room, just lying on the ground.
Bulldog 6 [Army Warehouses] - You won't be able to get to this weapon unless you assassinate the guard standing in the hallway to the armory, or if you lead the assault on the freedom base and then grab it before your platoon is overwhelmed. I'd recommend the stealth method but that's just me. Anyway, it's in a blue box on top of the lockers. If you poke around you'll find 12 grenades in the lockers that can be used with it. I used this weapon on the assault on the Freedom base, then when I ran out of ammo I turned it in to the Duty leader and got myself a sweet PSZ-9Md Universal Protection Suit (see below).
PSZ-9Md Universal Protection Suit [Duty Base] - There are two places you can get this, and it's kind of dumb how they set it up because you'll probably end up getting both at one time like I did. The leader of duty will tell you to go steal the Bulldog 6 grenade launcher and bring it to him. Do it and he'll give you the suit. There's another guy in the Duty base that will give you an assassination quest. Kill the traitor in the Freedom base and he'll give you a suit also. Put it in your closet and use it when your main suit gets a coffee stain or something, I don't know. This suit is the best one you can find in the game (beats the SEVA suit because it looks cooler!), so I suggest picking one up. If you joined freedom and can't get this suit, never fear, you can get a SEVA suit (read on)!
SEVA Suit [Yantar/Rostok] - The easy way to get this suit is to take Ghost's armor off his body and turn it in to the scientist in Yantar. He'll let you buy one for 30k RU. Since money in this game is absolutely pointless, you should be a freaking millionaire and that won't be a problem. If you're short on cash or accidently sold Ghost's armor or whatever, there's another way. You've got to kill mercs in the Rostok area until one gives you a treasure location for a safe inside a house. You'll know it if it's the one, it's a highly radioactive building and your gieger counter will be going off like crazy when you walk on the outside of it. The only way in is through a blown out hole in the roof, but watch your step because there's a pit full of electro anomalies that you can't climb out of.
Spring (Artifact) [Bloodsucker Village] - Get this artifact. As far as I have found, this is the only location for it in the game.. and it rocks. 30% Impact, with no side effects. If you do the Bloodsucker quest from the guy in the bar, you'll find the location of the Bloodsucker lair.. it's a big tower in the north-east corner of the village in question. Climb to the top and claim your prize.
Flash (Artifact) [Rostok, and near electro anomalies elsewhere] - You may want to pick up a couple of these. I carry two with me at all times. Each one gives you 73 endurance for a mere -10 electric shock. Awesome trade off. So basically, with two of these equipped, if you're carrying under 50kg of items, you will never run out of stamina. You can sprint permanently. If you're overloaded, your stamina will drop like it normally would (although I think it's a little slower), but the awesome difference is that it will recharge very quickly if you stand still, and it will still recharge while you're walking/moving.. whereas if you weren't using these artifacts, you would gradually lose stamina and have to stop.
M203 [Army Warehouses] - Can be found in the armory room of the main Freedom building. When you enter the first floor, take the hall to your right and you should see a gated doorway. Shoot off the lock and get inside. To your left should be a gunrack with a few LR-300s on it. One of them should have an M203 attached. Also, there is an M203 in Wild Territory, but it can only be found with information on a stash.
GP-25 [Dark Valley/X18] - Thanks to IceShade for helping me find this. Inside the X18 lab there is a large locker room (four sets of lockers put back to back). There is a gravity anomoly in a hole in the wall across from the locker room's doorway. Search the lockers and you will find a GP-25 attached to an AK74.
Bulldog 6 [Army Warehouses] - There is a second Bulldog in a village to the north/northwest of the Freedom base. In that village there are many Bloodsuckers so be careful. There is one house, with an opening in the floor with a ladder to the basement. In it you will find the Bulldog 6, grenades, and medkits. Alternatively, you can go with the Duty mission of retrieving the Bulldog and getting info about the Bulldog in the village from Chef. You have to be still friendly with Freedom to get the info.
Sniper modified LR-300 [Army Warehouses] - Northeast of the village with the Bulldog 6 you will find an encampment of Mercs. In one of the small buildings near the back of the camp you will find the LR-300 in a clamp-type thing. You can give this gun back to General Voronin of Duty to complete his mission and kill him later to retrieve the weapon. Apparently no one cares that he is dead.
"Storm" Abakan [Dark Valley] - One of the Stalkers in the Bar will give you a mission to retrieve a weapon he dropped when he was given a mission. Note: this is NOT the "family rifle" mission. The objective marker will point to a Bandit who now holds the Stalker's rifle. It is an upgraded Abakan with an integrated GP-25, which is very unique because the GP-25 cannot be attached/detached from a regular Abakan. Once you acquire the rifle from the bandit, stash it somewhere and tell the Stalker you won't do the mission. You will fail the mission, but it is more than worth it to keep the "Storm" Abakan.
Stalker Suit (early) [Cordon] - Yup, it's possible to get a Stalker Suit before you've even fired a single shot in the game. Here's how. In the starting village, go over to Wolf and climb the ladder next to him. Instead of going inside at the top of the ladder, edge round the corner so that you're standing on the house's roof. Now make a sprinting jump onto the roof of the next house along. Go to the far end of this roof and look for a hole, it's a small one and you can't climb in through it, but take a look inside and you will see a wooden box. Break it and pick up the Stalker Suit it drops (ok so I lied, you have to fire shots before you get the suit!. This makes early game so much easier, without actually cheating.
Rocket Launcher [Dark Valley] - In the building above the entrance to the lab here, head upwards. On one of the top floors there's another rocket launcher with one shot in it. Not remotely worth taking of course, but noteworthy nonetheless.
Mama's Beads (Artifact) [Bar] - Simple really, do the quest to kill the master stalker for the information broker standing next to the bar. Aside from the notably modified guns the master stalker and his two buddies carry, the quest reward will be the Mama's Beads artifact. It gives 5% bullet resistance with no side effect. I've also known this artifact to spawn around the area you start off in Dark Valley.
GP 37 [Warehouses] - If you do the quest for Duty to eliminate the snipers, be sure not to leave the base without picking one of these up. If you decide to join Freedom, pick it up from their merchant. It is the be-all and end-all of long range assault rifles (sniper modified LR300 is great but lacks close up firepower) - built in scope and incredible accuracy. If you're with Duty, use this gun to farm the spawns of Freedom soldiers to get regular 5.56mm ammo, if you join Freedom just do quests and buy AP 5.56mm ammo. Quite evidently a more desirable gun if you're with Freedom.
Tunder S14 [Warehouses] - If you're with duty, just pick up one of these when a squad-mate gets killed. If you're with Freedom there will be plenty lying around after the first mission. While the GP37 has it all at long range, this is the mother of all close range weapons. Excellent rate of fire, high damage and a grenade launcher. Better to grab this if you side with Duty, as it uses the rather rare 9x39mm ammo, who's SP6 (AP) variety you can get from the Duty faction easily.
Moonlight (Artifact) [Rostok] - This should be in the same place grinder has noted you can find the Flash artifact. It's name implies a night-time spawn, I've only found one of these artifacts, and I was in the area at night (didn't check the time, but probably close to midnight from how dark it was). This gives in the order of 106 Endurance, presumably the best of that line of artifacts! Well worth getting! The night-spawn thing could well have been a fluke, seeing as I've only actually found one.