Well, I'm a staff member over at www.playmoreconsoles.com and I have started to make a stock pile of wallpapers for a lot of different games. So far theres only a few, but there will be plenty more to come.

Grand Theft Auto: San Andreas
GTA: SA - 1024x768
GTA: SA - 800x600

Guilty Gear Series
Guilty Gear - 1024x768
Guilty Gear - 800x600

Half-Life Series
Half-Life 2 Concept - 1280x1024
Half-Life 2 Concept - 1024x768
Half-Life 2 Concept - 800x600

Doom 3:
Doom 3 - 1600x1200
Doom 3 - 1280x960
Doom 3 - 1024x768
Doom 3 - 800x600