Dark Reign
Heavy Gear 1 and 2
Mechwarrior 1 and 2
Nox
Revenant
Sim Theme Park or Theme Park World
Tomb Raider 3
Zork Grand Inquisitor

...among several others. Zork goes to highest priority though. Plus if you can get Heavy Gear to work in XP, that would be a godsend!

I'd love the original 1999 Aliens versus Predator - if GOG had it I'd buy 3 copies and wallow in sweet nostalgia of the multi-player co-op mode with two of my brothers.

EDIT: I'm an idiot - there's a version of this on Steam; how I missed it I don't know.

It's more difficult to count, but I'm the fifth of this topic to beg for :
Incubation : Battle Isle Phase 4 from Blue Byte, which is still alive, but was absorbed by Ubisoft.

Ok here we go:

Wing Commander: Privateer (tied with Ultima VII: The Black Gate for best game of all time, imo)

Freelancer

Star Wars Knights of the Old Republic II: The Sith Lords

Wing Commander I, II, III, IV, & V

Crusader: No Remorse

Any games from the X-Wing series

Worms II

Gladius (never released for PC but if only there were some easy way to port the Xbox version...)

X-Com series. The tactical games anyway; I don't think anyone particularly liked Interceptor or especially Enforcer.

Also, Dungeon Hack is a neat D&D graphical Roguelike from way back. I never did get very far but it's a fun game.

Lucasarts legendary adventure games (Grim Fandango, Indiana Jones and Fate of Atlantis, The Dig, LOOM, Monkey Island...), X-com series, Amberstar and Albion, Dungeon Keeper, Mechwarrior series, Legacy of Kain series, Metal Gear Solid, Final Fantasy 7 & 8, Blade Runner, Dreamfall, System Shock series... and from newer ones... I would go for "King Kong game" (if for nothing else that it is from Michel Ancel)...

SWAT 3 and Rollercoaster Tycoon.
You already have the publishers, so get to it GOG!

simgolf

alpha centauri

always check steam, i see many people request stuff they already have...

and for requests:
C&C generals and C&C zero hour (EA)
Nocturne (Terminal Reality)
American Mcgee Alice (EA)
Clive Barker Undying (EA)
The Thing (Vivendi Games)
Dark Colony (GameTek)
Noctropolis (EA)
The Operative: No One Lives Forever (Sierra)
No One Lives Forever 2: A Spy in H.A.R.M.'s Way (Sierra)
Enter The Matrix (Atari)

X-Com series are a must have.

Alpha Centauri is the main one that comes to mind. I'd also love to see Pharaoh and its add-on (the Sierra city building game).

Edit
Almost forgot - and this game takes first place for me, since the above two games would still work on my system but this doesn't, least not stably - is I Have No Mouth, And I Must Scream.

If you guys were to convince 3D Realms to bring some more of their old back catalog here, say, specifically Hocus Pocus, I would proceed to say very nice things about you to all my friends of wealth and influence.

And also my regular friends who actually, you know, have time to play video games.

New request/beg!

Anachronox - a 3rd-person, turn-based RPG. Just found out that the main designer of Planescape Torment said if he'd played it before he made Torment, he'd have put a load of stuff from it into the game - inspiration wise, rather than copy-pasting, of course.

Sad face that no-one has mentioned the old classic, Dark Forces?

*sigh*