It's free.

How about Homeworld?

It's be pretty sweet to see the PC version of Guilty Gear XX #reload pop up on here. With the release of Guilty Gear 2 and Battle Fantasia in the US recently and BlazBlue being set to release in a couple months in Japan, it seems like ARC System Works is getting a bit of a surge in popularity now, so managing to snag the rights for GG might be a good idea. It's currently being sold through digital distribution in Japan for not much more than $10 too, so I don't think the publisher would have many objections either.

How about some of the old BlueByte Games like Battle Isle and Die Siedler

The Elder Scrolls Daggerfall, and The Elder Scrolls Arena.
definetly Ultima 1-9.

Arena is freeware now.

Daggerfall: Fall of Aceras. Who didn't love the big open world that Bethesda does so well. SO much to do... I was actually afflicted with lycanthropy and became a were-boar when I played.
"You feel the need to hunt the innocent." and you only have 1 hp until you do.

The Adventures of Willy Beamish, ancient point and click adventure game. I never got to beat it cause my copy had a game crashing bug in it -_-

System Shock 2,primarily.

Would be a nice thing to have Constructor again....
The site is very well built. Congrats!
Bring our chilldhood back, GOG! o/

Sid Meier's Alpha Centauri
Seeing Homeworld here would be brilliant, maybe HW: Cataclysm as well?

Mario Teaches Typing

Old Bullfrog games (Theme Hospital, Dungeon Keeper...) will fit just perfect on GOG.

Mechwarrior 2 plus expansions.
I'm jonesing for it. I wasn't able to get it work on my computer even though I was able to install it. So a version that would work on XP/Vista would definitely be awesome.

I would love planescape: torment, one of the best computer RPGs I've ever had the privilege to play.