Hmm...the only game I can think of is Full Tilt! Pinball from Maxis. It was the game that came packed in with Windows '95 all the way through XP. I loved that game. But I could never find the whole thing, only one table was available for free. Since I now have Vista, even that isn't an option.

All the Jazz Jackrabbit games
Privateer II the Darkening (played that sucker for six weeks then lost all my files. Couldn't bear to play it after that.) If it was to return however....

MULE ++
Also, I'd pay $5 to play Panzer General again.

New to the forums and too lazy to look back at the gazillion suggestions . . .
I have a couple titles that spring to mind.
Dark Forces (the original FPS that got me into the genre)
Day of the Tentacle
Full Throttle
Blood
Blood II
Rise of the Triad
Red Baron
50 Mission Crush
Got a handful of others in mind -- Betrayal at Krondor, with the original soundtrack (was available briefly for free, but no sound. And wouldn't run on anything higher than Windows 98). Return to Zork. A few other RPGs.

Absolutely Rise of the Triad... well anything from Apogee

Game - Publisher or Developer - Release date
7th Legion - Microprose 1997
Blind Justice (gameplay was sucky, but it was still fun) - R.A.W. Entertainment - 1995
Constructor - Acclaim - 1997
Commander Keen series - id Software - 1990-1991
Cyberia 1 and 2 - Interplay - 1994 and 1995
Defiance - Avalon Hill -1997
Ice Breaker - Magnet Interactive Studios - 1995
Incoming (really fun game!!) - Interplay - 1998
Incubation + expansion - Blue Byte Software - 1997
Terminator: Skynet - Bethesda Softworks - 1996
Thanks for looking :)

Monkey Island please and thank you!!

Love to see Battle Isle 2220, especially a version that didn't randomly decide the computer could kill like a full force of my guys with 1 guy until I uninstalled and reinstalled the game each time.
Would love Populous as in the ORIGINAL but there really are no good PC versions to get running unless you go to EGA graphics :-(

absolutely Dungeon Master 1 and 2 :)

Battlezone
Pretty much everything else has been said.
Edit:
Ground control & expansion

You could always donate your money to the developer of the awesome Typhoon 2001 instead. It's a really stunning remake, and much slicker than the PC version of Tempest 2000 was, from the sounds of it.

Carmageddon
Any of the old LucasArts adventure games

I would love to see the Myth series come to Gog.
They were some of the best RTS games ever made back in the late 90's by Bungie.
Shame they sold the rights and take 2 will likely never use it again.
I would love to have a digital copy of the games to play when I wish without having to dig out my old cd's/cd key.
Would also be good to see a little community for it spring up, as the games were heavily multiplayer friendly and very expansive, with a ton of mods made for it, it was like the half-life of RTS games.
I would also love to see the Ultima series. One of my favorite rpg series of all time, even Ultima IX was decent once you patched it all up.
Then there are a couple of more diamond in the roughs that I loved back in the day.
Lands of Lore/Lands of Lore 2. These were two classic rpg/adventure games for me. They played out in the first person perspective like other rpg's back in the early day's, they had some interesting characters and storylines that tied in well.
Then there's my two favorite Melee games of all time.
Severance (aka Blade of Darkness) and Rune. I give the nod to Severance for having the better combat system (it actually requires you to be defensive, block/dodge and not just ho-hum it like most melee fighting games do), however rune had a great storyline/atmosphere to it and the viking theme made it very memorable.
Multiplayer in rune was so much fun, playing in tihe arena's and throwing a sword at someone's head hah, will never forget it.
Then there's a little unknown game that I doubt hardly anyone played,
Dark Earth.
It was an adventure styled game with a memorable setting/atmosphere. The characters and storyline were quite well done and it got fairly good reviews, however it flew under the radar like sacrifice and other games have done in the past.
I also would love to see some classic adventure games.
Sanitarium
Day of the Tenacle
Grim Fandango
Monkey Island
Sam and Max
etc.

Whoah! I just remember a game I can't find anywhere anymore, not even on eBay: Rage of Mages (1 and 2).
How I would love to play those two again :-(

Just visit 3Drealms, you can buy it there ;-)