Vampire The Masquerade : Bloodlines.

+1 to that. I'd love to play the HD version sometime. I have very fond memories of the original from my Amiga days.

I would instantly buy:
The Beast Within
Discworld Noir
I would instantly buy:
The Beast Within
Discworld Noir

I remember it, but I only played the demo, back then (I also played the demo for Dungeon Keeper at about the same time). I liked it, though I never got the chance to buy it (it was never released on my country).

MechWarrior 2
Homeworld
MechWarrior 2
Crusader: No Remorse
MechWarrior 2
System Shock
Oh and maybe MechWarrior 2

Ultima IX: Ascension - Dragon Edition (contains ultimas 0-9)
I'm willing to donate several of these to Ultima fan projects if this edition comes to sale.
Dungeon Siege: Legends of Aranna
funnily I'd probably donade few to Ultima 6 Project team first
Only problem in instabuying right now is that many of my yearly bills will come in first three months of the year and money is already tight after christmas. By april my finances should settle down so I can affort some impulse buys.

That, too. Also I wouldn't mind some 31st Century Combat.

Shame the third game of trilogy (crusader: No Mercy) was never completed.

This topic is a particular case of "games I'd like to see on GoG". :)
Anyway, for me the instabuys would be:
- HoMM Chronicles (all pack)
- Little Big Adventure 1 and/or 2
- Quake 3 arena + team arena
So far I didn't intabuyed any game. Ad that's mainly because I lack the necessary time to play the games I already have and I'm patient.

How is MAG? I heard that people are having network issues and stuff seems really broken for a multiplayer-only game.

Anyone know if it has any DRM? It's 8.99 through metaboli... but i don't know if they use DRM as standard or not.

I've been able to play online just fine, and the action is pretty fun. I'm not sure what people would say is broken about it. One weird thing is that at the beginning, you choose a faction for your character and that's it. You can only have one character, so if you want to change factions you need to either delete your character and make a new one or make another PSN account.
They organize the 256 player chaos in a productive fashion, so each group is working on their own goal, contributing to the overall success of your side. The guns have a nice kick to them, and the healing devices feel right. I've enjoyed playing it a lot more than Modern Warfare 2.

Hmm, I can't remember many instant buy games for me -- although I'm sure they're have been a few more. But ones I do remember are:
Ico (PS2)
Wanda to Kyozou [Shadow of the Colossus] (PS2)
Toppo wo Nerae! Gunbuster [Aim for the top! Gunbuster] (PS2)
Mirror's Edge (PC/PS3)
The ones that I would buy in an instant if they ever appear (here or elsewhere):
Eye of the Beholder series (PC)
Hitokui no Oowashi Trico [The Last Guardian] (PS3)
Elite (PC)
Freelancer (PC)
Settlers 4 (PC)
Laser Squad (PC)
And probably a few others I don't recall offhand.

Dungeon Keeper 1 & 2
System Shock 1 & 2
Homeworld 2

Planescape Torment (for me and for a few other people)
Other Infinity Engine Games
Elite series (couldn't buy them seperately but as a whole they'd be worth it)
Crusader series
Wing Commander series
Ultima series
Grim Fandango (have it already but its made of awesome, same as PST I'd get it for others too)
Discworld Noir (just to have the bugger working)
TIE Fighter (also the other SW space combat sims but TIE is Emperor of all)
Loads of others I'm sure