I would love to see Ultima 7 and Ultima 7:2 Serpent's Isle.

I cannot WAIT to see the Mechwarrior games uploaded....if GOG are going to do the activision cat then they'll be mad if they don't do MW2 or MW mercenaries.

Planescape Torment. Always wanted to play it, and never really got around to buying it. It would be an instant purchase if it showed up here.
Urban Assault. I found the demo of the game strangely captivating, and would love to take a stab at the full game.
Raptor: Call of the Shadows. Having a version of this game that runs without hassle on my modern PC would be simply fantastic.
Thief Gold and Thief: The Dark Project. Again, much like Raptor, having versions of these that run on my PC without a ton of configuration work would be excellent.
System Shock 2. Instant purchase. I seriously regret not getting this game when it was at retail, and would pounce on any opportunity to get a legitimate copy.

GOG is amazing. DL'd HOMM3 today, and there was much rejoicement.
That being said, I'll now name a few other instant purchases would be:
monolith's BLOOD
I never got it working with DOSbox, and as such I've missed it immensely since I left win98 behind. It's abandonware, but I would gladly pay to just get it smoothly working on modern computers! Wonderful game.
Conqueror AD 1086, Sierra?
Cheesy game, but It'd be nice to relive that particular part of my early youth.
ID's Hexen and Heretic
Grim Fandango, Lucasarts
and ID's more recent Return to Castle Wolfenstein. It's not that old, but it's old enough to be hard to come by. At least in Sweden=p
Cheers!

Personally I woudl ike to see
System Shock 2
Icewind Dale series
Because I never really got to try them but seem like pretty good games.

Check out this guide I wrote for FPSBanana.com to get Blood working on dosbox
http://sebby.hostoi.com/legacy.html (would actually link to the FPSB tutorial, but the site is choking like crazy...like always)
and you can buy Return to Castle Wolfenstein through Steam - it's still $20 though...
http://store.steampowered.com/app/9010/

I would love to see Cossacks: Back to War on the site. I love that series, but it can be really hard to find and European Wars has issues with working on newer machines. I'm also throwing in my hat with the Icewind Dale games, really fun dungeon crawls and probably some of the best isometric rpg combat. The only other that I think would be really cool is Crimson Skies, arcade-y flight sim with a great setting.

Sebhael:
Thank you for the tips! I'll look into it.
Still, should GoG buy/release blood, I'd buy it ;)

yeah cossacks is great, but i have no problems running back to war on vista. if we would get cossacks we would get other great games from cdv...

I already posted them on other threads, sorry, but I want those games with all goodies dammit.
Prince of Persia Warrior Within (I have issues with Ubisoft, but dammit, I won't hesistate to buy that again)
Splinter Cell Series( like above)
Cold Fear(again, like above...dammit ;_; )
Max Payne series( I hope Microsoft won't say no, especially after the Alan Wake crysis)
American Mcgee's Alice
System Shock series
Lucas Art's Click and Point adventures (LOOOOOOOOOM)
Devil May Cry 3
Resident Evil Series
Syberia series
Obscure Series
Black Mirror
Penumbra Trilogy
Silent Hill Series 'til 4
Jordan Mechner's Classic Prince of Persia trilogy
Metal Gear Solid 1 and 2 (yes, they are ported on PC, I even have a copy of them)
If it's possible, Braid... please ;_;
EDIT: Found out that Parasite Eve 2 was ported for PC:
http://www.gamerevolution.com/preview/pc/parasite-eve-2
Please, GoG, add it aswell along the former game!

Don't know if it was mentioned already, but I wish for "MAD TV".
I know it's available for free, but it does crash regularly for me with Dosbox.

THere are a few I would add my voict to wanting...Dungeon Keeper, and Planescape Torment. Both are very hard to find and expensive when you do.
I have both, and GOG would not have to do a lot of work since the original version for both actually don't have many problems on my modern computer. (Of course I am using XP and it might be different for VISta or Windows 7. Still, no problem for DK there since it came in both DOS and Windows.)
I would LOVE to see the whole General Series of Games from SSI on GOG. The Panzer General Series and it Sci Fi/Fantasy spin offs, Fantasy General and Star General.
BTW there is a official patch out for Cossacks:European Wars that solves the problem with it running on modern computers.
BTW I am betting we will not see any Lucas Arts games on GOG. I have a feeling they have an exclusive deal with Steam.

If LucasArts had an exclusive deal with Steam, none of their games would be available from...say...Direct2Drive. And they are.

deus ex: GOTY !

Watch me, I'm begging!
Please dear GoG, release in the near future at least one of the following:
Quake III: Gold
Little Big Adventure
Little Big Adventure 2
Lotus: The Ultimate Challenge
Heroes Chronicles series
please... :D
Ok, I feel better now.