List the games that you have instantly bought, or that you would instantly buy, as soon as they were to become available. Mostly old games, except a new one if its good enough to induce a major hysteria.
Here's my list, including some obvious ones but not necessarily complete. No particular order.
Tribes 2
The Quivering
Fugger 2
Flashback
Another World
No One Lives Forever
No One Lives Forever 2
Flatout 2
Blackthorne
VVVVVV
TETRIS!
more obvious ones: Psychonauts, Sam & Max, Planetscape Torment, both Fallouts, Oddworld, Sacrifice, Earthworm Jim, Beneath a Steel Sky, Beyond good and evil, Jagged Alliance 2, Commandos 2,3, much of the top 10 from the wishlist...
I'll think of some more. What are your gems?

No one lives forever : the only FPS since Hexen that kept my attention.
Planescape : I already have the game, but I'll snatch the digital version no matter what the price will be.

Is it just coincidental this instabuy thread was greated by a person named instant?

you DO realize beneath a steel sky is a FREE game here on gog already?

All the IE games at any of the current price points (I'll be bold and say "anywhere below $15", would probably accept up to $25 if I didn't already have legal copies of them); Baldur's Gate, BG2, Icewind Dale, IwD2, Planescape.
* I have them all
* They work surprisingly well in Vista x64 and Win7 x64 (BG1 has troubles creating or joining a multiplayer game while in fullscreen for some reason, and PS:T wants to switch to 16-bit colour, other than those I have found no problems)
* My homemade mini image alleviates the problem of needing the discs for any of the games
* I could easily create my own installer for them to not even having to bother with swapping discs and patching as part of the install
... Still, I'd buy them as soon as I see they're available here.
Others, that depend more on the price, are Age of Empires 1 to 3, Syberia 1 and 2, Warcraft 2, Diablo 1, Zanzarah, Dreamfall, Sam & Max... uh, there are probably others I can't remember off the bat as the lis has become too long (yes, the list of instabuys, not to mention the list of old games I probably want to play some day). List does not include ones I've already bought from here.

Another world official HD version:
http://www.anotherworld.fr/anotherworld_uk/index.htm
PS/ Gog should really contact the author and get him onboard as soon as possible. As a one-man deal it should be something they can put together in a weekend....

I think the only title I've purchased immediately upon its release was the Might & Magic collection.

Would buy: Syndicate, System Shock 2, and a fully fixed up or remade version of Bloodlines.

I insta-bought Commandos Ammo Pack, and Constructor.
I'd insta-buy quite a few games ranging from :
Diablo 1 (with or without Sierra add-on "Hellfire")
C&C 1, 2 (hell, any chance of Decade Pack now it's 3 years old or so? :P )
Theme Hospital
Archimedean Dynasty (if it is the USB analogue joypad that's the ssue I can't get it working properly, and it's fixed :P )
Civ 2
Mechwarrior 2
Total War - at least, Rome
Quake 2
Starfleet Command 1...maybe the other 2, too.
Vampire : Bloodlines
Neverwinter Nights 1
Armored Fist
Future Cop LAPD (Speaking of which, wasn't Nuclear Strike released for windows?)
Darklight Conflict (Anyone remember this wing commander-esque game? mid-late 90s)
KKND
UFO!! Enemy Unknown, and Terror from the deep....well, maybe insta-buy
Privateer 2 - Maybe Wing Commander 4/prophecy, too
Warcraft 2
Gah, my bank's gonna love me if I go overdrawn buying all those! I better stop now :P

Operation Flashpoint ... I've learned my lesson LOL

I guess the only one would be Grim Fandango; I love that game and have been searching for a decently priced boxed copy for a long time now.

System Shock
WarCraft II: Tides of Darkness
Blood
Thief II: The Metal Age
The Elder Scrolls, Chapter II: Daggerfall
Strife
God of Thunder
Own them but still would pay for them, even the $9.99.

I recently insta-bought MAG and Mass Effect 2. Good games. Oh yeah, I also did the same with Gabriel Knight I as soon as it was released here.

Vampire: The Masquerade - Redemption

That just makes it even more instant.