You totally have to release Planescape: Torment and Dragonriders: Chronicles of Pern and Star Trek: Deep Space Nine - Dominion wars.
Oh so pretty please! I can't play PST on vista because gametap doesnt work on vista 64, chronicles of pern doesnt work on NT systems and DS9 doesnt really work that well on xp either.

Planescape Torment. I've heard so much praise for it, but can only find the discs being sold at exorbitant prices.

System Shock 1 & 2
Day of the Tentacle
Monkey Island games

Oh I almost forgot, the Baldur's Gate series along with the Icewind Dale series
Maybe even the Age of Empires series!

Indiana Jones and the Infernal Machine
That is great action adventure game.I would love to see it on GoG. :)

I just thought of another one, not sure if someone has already posted it though. There was an adventure game called Deja Vu back in the late 80s. I played it on the NES, but it was released for DOS as well. I'm not sure how different they are, but the NES version was great stuff. They also did a sequel on the NES, but I never played that nor do I know if it was released on DOS.

Old Origin games would be wonderful, the wing commander series and such... Maybe some really old games like the Ultima and Wizardry Series, maybe the Bard's tale games.
What I am really hoping and wishing for though are Crusader: No Remorse and Crusader: No Regret!

The Infocom games
The old id games, Wolfenstein and Doom
M.U.L.E.

Oh, I just thought of one. It's an old Macintosh game called Uninvited. It's a great game. It might have been on DOS too, but I don't know.

I thought I would re-iterate some of the games already said, to show increased demand
planescape: torment (NEED!!!!!!!!!!!!!!)
syndicate, syndicate wars (NEED!!!)
alpha centauri (NEED!!!!!)
stars! (NEED!!)
epic megagames, appogee classics
the old leisure suit larry games
the old indianna jones games
bladerunner
omf 2097
slave zero
shadow warrior
heroes of might and magic series
revenant
re-volt
dark colony
sanitarium
ANY AND EVERY SIDE SCROLLING GAME

Oh god, if ever there was a loaded question it's this one.

Don't know if it's been requested yet, but how about Ceasar 2? I loved that game, especially hearing "Plebs are needed" over and over. :)
The other one I would like to see is Spacebucks. Loved that game when I could play it. :)

Maybe the old Police Quest games by Sierra. I really enjoyed the original keyboard only game, not the updated point and click. Also, Grim Fandango and the other Lucas Arts (if they would allow that). A long time ago I played a demo for one of the Ringworld games that seemed like it would be fun. Never did get a chance to grab it. 7th Guest and 11th hour as well. I have the original dos games, but sadly, nothing old enough to play them on.

my votes:
space quest series
kings quest series
police quest series
any x-wing or tie fighter game