First of all, I'd like to give you a little backstory on S.T.A.L.K.E.R: Shadow of Chernobyl. It's a survival FPS with a few minor RPG elements, made by the Ukrainian developers GSC Gameworld. It's based off three different things, the Chernobyl NPP disaster, Roadside Picnic, and the movie Stalker. In a world where the Zone of exclusion was hit by a second nuclear disaster, many people entered the highly dangerous zone for different reasons, but mainly to pawn off artifacts and be a badass. You are Marked One, a rookie in the zone with Amnesia. You help out the local trader Sidorovich, and eventually find out some serious stuff about the zone.

This game would be amazing vanilla, but it was released with too many bugs to count. And these mods will make the game so much better.

First off, arguably the best graphics mod. Shadow of Chernobyl: Complete. It gives a big boost to the graphics, edits all the weapons, and makes the zone more enjoyable. It's recommended that you play this in your first playthrough.
http://www.moddb.com/mods/stalker-complete-2009


Next is the big bad mod meant to be played by hardcore players. This is one of the oldest, and still one of the best mods for SoC. Anomalies are invisible, enemies are stronger, you get less money, random blowouts occur, and cut mutants from the game are added in. Don't ♥♥♥♥ get Oblivion Lost, it ripped off AMK.
http://www.amk-team.ru/forum/index.php?showtopic=480

L.U.R.K is similar to AMK, but also gives the player an annoyingly unforgiving time. Invisible anomalies, blowouts, no HUD, reduced weight limit, no artifacts, hard AI. This is the best total conversion mod out there. And like Complete 2009, it has amazing graphics.
http://www.moddb.com/mods/lurk

SMP: aka Super Mod Pack. Includes many small mods, combining them all into one super mod. It adds in a lot of bonus content, and reworks a lot of vanilla content. NPCs are more realistic, i.e friendly stalkers help you out. Loads of weapons. Faction wars, they will constantly try to take over control points around the zone. A spawn menu (holy ♥♥♥♥), and additions for /k/ browsers, (Mosin Nagant, /k/ommando gatling gun.)
http://www.moddb.com/mods/super-mod-pack

Priboi Story: a total conversion mod, you are Lieutenant Priboi Slipchenko, and you were sent to the zone. Takes place one year after the actual game. Awesome story if you want more content.
http://www.moddb.com/mods/priboi-story/ ... -story-v11

I've been meaning to look into that. I'll try one or two of those out once I get thedone in the vanilla game.

It was incredibly disappointing when I couldn't finish the game on my old computer because the very last and most challenging portion of the game before the was the most computer intensive.

After I beat the game for the first time, I went back to do the bad ending. I experienced lots of game bugs. Such as: getting hit by a rocket, flying off the screen, and floating in a grey space. Also, the bad ending is a challenge. Especially if you go in with no ammo.

When I was attempting it on my old computer, I had very little ammo, no meds, no antirads, shredded scraps of an exoskeleton, and loads of lag, and that's where I called it quits, until I played through it again recently.

It makes a world of difference to have a few good antirad and health regenerating artifacts when you go into the latter part of the game. And understanding that health +400% only means 500% health regeneration rate helps a lot too.

I usually get a couple mama's beads, and some endurance artifacts so I can zoom past all the snipers.

Could use more descripions. I, for one, don't support using Complete for your first playthough but whatever. SMP is a horrible unbalanced mess of oversized textures, 8GB RAM limit, ♥♥♥♥ that doesn't even fit in the universe and a million AK reskins. If you want to get a megamod mishmash try the Russian megamods like Secret Trails 2, Narodnaya Solyanka, NLC, and so on.
Oh, and you forgot ZRP (way better for a first play than Complete).

Great, now I want this game, its 20 dollars on steam and I only have 15...

NOSTALGIA ATTACK!
Commence installation of Stalker trilogy.

So tell me about ZRP

It's what Complete stole most of the bugfixes from. It actually retains vanilla gameplay unlike Complete. No graphics changes, no easier difficulty, just fixes.

I like all of the extra features Complete sports though. Especially the post game freeplay.