Before S.T.A.L.K.E.R. was conceived GSC made another FPS named Venom (Codename: Outbreak in the west) based on another Sci-fi book called The Puppet Masters by Robert A. Heinlein. The game shares some similarities with S.T.A.L.K.E.R. in that it has fairly open (though linear) missions and also has an RPG-like interface and inventory. Unique to the game is that the player has only one weapon that can transform into a shotgun, laser rifle, machine gun, etc. depending on the situation. The game is also two player, either with another player in cooperative mode or with the AI. The AI's view is presented in a little window on the player's screen. It is very hard, if not impossible; to get Venom working on today's PCs.

Firestarter was actually the first game to use the X-Ray engine built for S.T.A.L.K.E.R.. Although published by GSC, Firestarter was developed by a team entirely separate from the S.T.A.L.K.E.R. team. The game is a single player FPS in the style of Quake and Unreal. The music to the game was composed by MoozE who also composed the ambient tracks for SOC and is freely available here:

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The leads responsible for Venom, Sergey Zabaryansky and Roman Lut left GSC to form Deep Shadows and went on to make Boiling Point (originally known as Xenus); a completely seamless open world FPS set in South America that shared many design ideas with the original Oblivion Lost (the alpha build of S.T.A.L.K.E.R.). The game has many factions, who the player can do jobs for and increase their influence with. The game world is full of cars, boats, planes, and wildlife. Deep Shadows followed this up with Xenus 2: White Gold, but this was only released digitally in the west.

Andrew Prohorov, lead designer on SOC, left with the X-Ray engine programmers shortly before SOC's release to form 4A and make the Metro series of games. Metro 2033 is set in a post-apocalyptic Moscow where everyone lives in the metro train lines because of the radiation outside. Metro shares similar combat, harsh conditions, mutant enemies and environmental hazards in common with S.T.A.L.K.E.R.. Owing to its confined environment however, it is a much more linear and scripted game.

Vostok Games was created out of the ashes of the S.T.A.L.K.E.R. 2 team and is using ideas from the development of that game in their new game Survarium. Although right now it exists as an open beta team deathmatch FPS, Vostok games hopes to make Survarium a fully fledged open world game with a story of its own. Only time will tell how well they live up to the S.T.A.L.K.E.R. legacy, although the "freetopay" direction they are taking the game in at the moment does not bode well.

This is a list of mods I have played and enjoyed or believe to be significant; it is by no means definitive or complete.

Most of these mods will require a certain patch version of the game they are for (usually this will only apply to SOC). You will find this information in the mod readme. Players with the Steam or GOG version of the game may find this a problem as their game will be prepatched but there are ways to downgrade versions (google "downgrade SOC to *version*").

A word on the "Complete" mod: an overhaul mod that was released for each game. Many people will say that they skipped playing the out-of-the-box game and played with Complete installed to get the best experience. Frankly these people don't know what they are talking about. The SOC Complete mod makes the graphics brighter and prettier (not really a plus in a radioactive wasteland), the AI dumber, the game easier, and introduces as many bugs as it fixes. The Clear Sky Complete mod turns everything off (grenades, faction wars) without fixing anything and the COP Complete mod has no significant difference from the (completely fine) vanilla experience. In my opinion the Complete mods are superfluous and have been superseded by better overhaul mods anyway.

Shadow Of Chernobyl

Zone Reclamation Project (ZRP)

Most mods alter the base game in some fundamental way. The ZRP mod is significant in that it only fixes bugs in vanilla SOC. Playing with the ZRP mod installed would give a very similar experience to playing the original unaltered game, except without the remaining bugs.

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AMK

One of the first mods for SOC, AMK introduces blowouts, random anomalies, reworked ballistics based on realism, realistic dark nights, and a sleep system. The mod's biggest change is that it unlocks the A-Life system so that NPCs are completely autonomous; they can get in fights, call for help, loot dead stalkers or hidden stashes as well as sell the contents. The player is more likely to run into powerful mutants at random in AMK than the vanilla game. The mod generally makes the game much more of a challenge. There are many different versions and variations of AMK, the latest official version is 1.4. If you want an even more unhinged experience try the Oblivion Lost mod (lastest ver. 2.2), a derivative of AMK.

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Stalkersoup

Stalkersoup or Narodnaya Solyanka is a melting pot of different mods stacked on top of one another. This Russian mod is full to the brim with extra quests, items and enemies. Much of the content makes the game unbalanced, unfair, or both. Stalkersoup is not a mod you play to finish the game, but to have an experience. It includes every area from the three stalker games plus maps that were unreleased stuck together as one big game. The possibilities are endless in this mod. Techno bacon's release of the mod is the most stable (for a mod that is famous for being extremely unstable) and is all in English, though some of the new quest dialog can be hard to understand.

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Autumn Aurora 2

Autumn Aurora 2 is a very recent release and it shows as this is the best looking mod for SOC hands down. The mod gives the Zone an autumnal look and feel. It adds lots of atmospheric effects such as AtmosFear weather (see below), fog, and new nature sounds. It is not just an aesthetic mod, but also adds new features to the game. For the first time the Clear Sky / COP anomaly detector system is implemented into SOC, as well as a change in the UI and HUD to something more like the one from COP. It has model swaps for weapons, character models, and sights which all look in keeping with the theme of the mod. The economy is made more realistic as good weapons are harder to come by until late in the game and not many items sell for much at all. The difficulty is very similar to AMK. Underground labs are given a revamp with many more monsters than in the vanilla game, something that not many mods tend to change. All mutants have new sounds and are much more frightening as a result. The only criticism that could be levelled at the mod is that with all the shaders and fog effects it can be very hard to distinguish enemies from the rest of the environment.

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Clear Sky

Sky Reclamation Project (SRP)

There are no really important mods that I feel comfortable recommending for Clear Sky apart from fixes for grenade spam, pistol sights, and bug fixes. The Sky Reclamation Project, like the ZRP (above) is a mod that fixes all this stuff without altering the base game.

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Call Of Pripyat

Massive Simulation Overhaul (MSO)

MSO comprehensively overhauls the A-Life NPC interaction system in COP. It not only improves AI behaviour but also fixes how they spawn. I would recommend this for even a first time playthrough as it fixes lots of other little bugs as well.

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AtmosFear

Also released for Clear Sky, this weather mod adds beautiful sky textures and effects for lots of different times of day and weather states. AtmosFear also adds blowouts, fallout from said blowouts, and even psi-storms. Most overhaul mods include some version of AtmosFear (Autumn Aurora 2 is the only SOC mod I know which uses it).

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Also check out Crommcruac's Absolute texture packs for all three.

Misery

Misery is an overhaul/compilation mod aiming to increase the difficulty and survival aspects of S.T.A.L.K.E.R. to the extreme. The mod also has an art style that is even deader than Autumn Aurora 2. The mod adds lots of antiquated WW2 guns and weird items such as cannabis, and Mein Kampf. Every item has a use (the cannabis to combat psi-energy attacks, Mein Kampf to clean your weapons) but it can get a bit stupid when you're looking for a particular item or status effect and there are literally hundreds of items to choose from. Many elements of the game have been altered to be just plain unfair and impossible to get past; bloodsuckers (invisible "vampire" monsters) no longer make any noise and will kill you 100% of the time unless you happen to be facing them when they attack and one-shot them. Some difficulty balances do make the game more interesting, particularly in regards to the economy and the new hunting and cooking mechanics but the mod is still fundamentally broken and cheap as of version 2.1 without custom tweaks and fan patches.

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YouTube - Chernobyl Uncensored - Documentary
A very in depth documentary of the Chernobyl disaster and its subsequent cleanup and effects. Has interviews with experts, journalists, and cleanup crew members.

YouTube - Stalker (Сталкер) (1979) trailer
A short trailer showing many scenes from the Russian Stalker film put together by bioreflex.

YouTube - S.T.A.L.K.E.R. Build 1097: unfinished and test levels
YouTube - S.T.A.L.K.E.R. Build 1098 (February 3, 2002)
Video captures of unearthed early betas showing environments completely alien to YouTube - S.T.A.L.K.E.R.'s world (more in the channel).

YouTube - Making of S.T.A.L.K.E.R. - Clear Sky (Part 1)
YouTube - Making of S.T.A.L.K.E.R. - Clear Sky (Part 2)
YouTube - Making of S.T.A.L.K.E.R. - Clear Sky (Part 3)
A series of developer interviews into the making of Clear Sky.

Transcript of an extensive podcast interview with Andrew Prokhorov lead designer on SOC

Interview with Anton Bolshakov, who oversaw all three S.T.A.L.K.E.R. games, just after SOC's release.

Another interview with Anton Bolshakov.