GET IT GET IT GET IT
The way people talk about stalker, you'd think it was a mess of a game that requires a rare, specific taste to actually get into. I only got the first one because it was $5 on steam.
HOWEVER, it quickly became one of my top three favorite games of all time, and I have been playing assloads of pc game since I was five years old. I have seriously been obsessed with the entire series since about my fifth hour into SOC. It is the only game I have ever made art of and I couldn't get it out of my head at work or school. I have put over 100 hours into the series, far more than any other game franchise. I have spent hours reading about Chernobyl and Tarkovsky's Stalker movie and Roadside Picnic.
It is a singularity, I won't try to describe what's so great about it because there isn't anything to relate it to, you have to try for yourself.
tl;dr if there is even the slightest chance you will get nearly as much enjoyment out of Stalker as I have, GET IT.

Wow. OK then. :p
I've got Shadow of Chernobyl, which I couldn't really get into at first, but when I tried it again with the Complete and Realistic Weapons mod, I ended up quite liking it (I finished it a couple weeks ago). In the end, it's a pretty great game, and one that I enjoyed a lot more than its fellow open-world wasteland shooter Fallout 3, but the upgrade did a lot for it.
The question then is whether the expansions are worth a buy-in. Clear Sky seems unpopular in general, and I fear aggravating technical problems in both (though there is a Clear Sky Complete available).

I'm seconding his recommendation and adding that you should probably play them with the Complete mods (ideally with a weapon balancing mod too so the guns are actually dangerous). Pripyat Complete is apparently in the works too. I need a weapon mod in the meantime though.

D2D: Football Manager 2010 65% off ($13.95)

COP is the pinnacle of the series, I have put the majority of my Stalker time into it.
Good hunting, stalker!

Not a games only, but I wanted to share this tip with you GOGers. Amazon has two good sales running for this week, especially the first one for games and blu-rays:
"Gold Box Event: High-Def Week, June 7 - 11"
http://www.amazon.com/gp/feature.html/?docId=1000372481
"3 Days of Deals for Dad: June 7 - 9"
www.amazon.com/fathersday
EDIT: Games for the week:
Nier PS3
Nier X360
Just Cause 2 for PS3
Star Ocean: The Last Hope for Xbox 360
Final Fantasy XIII for the Xbox 360 and PS3
Just Cause 2 for the Xbox 360
Mass Effect 2 X360
"Celebrates the single largest sporting event on the PS3 and Xbox 360"
BFBC2 X360
BFBC2 PS3

Gamersgate late to the party: Splinter Cell Elite Echelon Edition 39% off ($57.48)

I added the S.T.A.L.K.E.R. 3 pack to my wishlist at GamersGate but will hold off a few days before committing purchase. It's possible Direct2Drive will try and match the offer, or best it before then. And if they don't then thank you GamersGate.

Steam: Batman GOTY will go on sale, probably.
EDIT: Batman: Arkham Asylum GOTY 50% off, $19.99

Blah. Yet another game Square Enix has decided those of us in Japan aren't allowed to buy on Steam. Bastards.

" Blah. Yet another game Square Enix has decided those of us in Japan aren't allowed to buy on Steam. Bastards."
*side note*
Speaking about region restriction.Got an answer from an indie developer yesterday. http://ghook.speedrungames.com/index_en.php[/quote[/url]]

PoP: Sands of Time
PoP: Warrior Within
Only $2,50 now on GFWL!

Is it DRM free?
If yes, GET IT GET IT GET IT GET IT!
It's the same as captfitz with Stalker: One of my 3 favourite franchises.
GET IT!

I can't see them listed here. Must be a regional restriction, which is typical enough of Ubisoft.

http://www.microsoft.com/games/en-US/Games/Pages/princeofpersiatsot.aspx
http://www.microsoft.com/games/en-US/Games/Pages/princeofpersiaww.aspx