(WALL OF TEXT WARNING)
(List is not in favourite order)
Great Zombie Games:
Dead Rising - Ok, this one's leet (360 EXCLUSIVE, it's got a Wii port though)
Left 4 Dead (PC, 360 and PS3)
Resident Evil 4
Resident Evil 5? - Never played, but looks awesome

(360 AND PS3 ONLY)
Halo 3 - Hey, it's got Infection game mode! <_< (360 EXCLUSIVE)
CSS Zombie Mod - Awesome (PC ONLY)
Garry's Mod Zombie Mod - Not as awesome as CSS's but still very cool (PC ONLY)
Call of Duty World at War - That "epilogue" mission

Epic sauce
Doom - I think it got zombies.. oh wait, it's got Demons! Whatever. (PC ONLY?)
Great First Person Shooters:
ALL Half Lifes - All awesome. Every single one is avalible to PC I think. And some to the PS3 and 360.
Counter-Strike Source and 1.6 - Awesome.
Team Fortress 2 - Best freakin' FPS of all time (to the PC)!?!??! I THINK SO! *awesome face* (PC is updated frequently, 360 and PS3 don't have any cool updates such as unlockable hats and weapons)
Crysis - Erm... It's got a nice multiplayer! xD Plus the Graphics are stunning. Looks like some sort of Halo 3 for PC.
Halo 3 - NOT the Best FPS cuz it's overrated as shit. It's still a very nice game. (360 EXCLUSIVE)
Halo Custom Edition - A totally moddable Online-Multiplayer-Only Halo Combat Evolved. VERY nice. (PC ONLY, broken by one of IE8's updates)
Halo Combat Evolved - Same as Custom Edition but it's got Single Player and Multiplayer and it's NOT moddable. (PC AND XBOX ONLY)
Blockland - Non-Linear Sandbox AND Deathmatch minigames, some addons allow you to have a Zombie game

(PC ONLY, YOU CANNOT WAREZ DOWNLOAD THIS)
Call of Duty World at War - It's nice.
Call of Duty 4: Modern Warfare - It's nice.
Call of Duty - BEST COD EVAR?
Doom - Best classic FPS evar?
Battlefield 1942 - WW2

(PC ONLY?)
Battlefield 2 - No comment (PC ONLY?)
Obsidian Conflict - HL2 co-op, bettar than Synergy in my opinion :3
Must not forget... FALLOUT THREEEEEEEEEEEEEEEEE!!! :hurray:
Great non-linear or yes linear sandbox games:
Garry's Mod - srsly, do I have to explain? (PC ONLY, NO STANDALONE)
Blockland - Online legos with some new bricks. (PC ONLY, YOU CANNOT WAREZ DOWNLOAD THIS, AS IT IS IMPOSSIBLE)
Great Racing Games:
GRID - Realistic, stunning graphics, and niec carz :3
V-Rally 2 Expert Edition - Ok, this game is OOOOOOOOOOOOOOLD, but it's AWESOME!! (PSX and PC ONLY, WORKS IN WINDOWS XP, UNCONFIRMED IN VISTA)
Destruction Derby - EXTREMELY OLD but really fun to play (DOS ONLY, USE DOSBOX PLEEZ)
Destruction Derby 2 - Same as DD1, but even funner, and a littel littel bit better graphics, more derby arenas and tracks, cars are different but the same type of naming: Rookie, Amateur and Pro. (DOS, WINDOWS 95 AND 98 ONLY, USE DOSBOX, CRASHES ON WINDOWS XP)
GTA San Andreas - Erm, it's got racing! and SA:MP racing is cool too... Can't wait until I get IV for the 360 to see how GTA Race is :3
Burnout Paradise - Epic Sauce (360, PS3 and PC)
Need for Speed Most Wanted - EPIC SAUCE (PS2, 360, PC, XBOX, maybe for the PS3 too?)
Need for Speed Carbon - It's nice. (PS2, 360, PC, XBOX, PS3)
Midnight Club (Forgot the rest of the name but it is for the PS2) - Epic
Gran Turismo 2 and 3 - :bow: (PSX only?)
Flatout Series - 1 was good, 2 was great and Ultimate Carnage OWNZ
Ford Racing 3 - Super Unrealistic but really fun

Daytona USA - Shit port for the PC but it's still fun...
Great Platformers:
Sonic 1, 2, 3 - Better than the new ones! (Sega Genesis and Mega Drive ONLY, avalible in the 360 Arcade Marketplace)
Jazz Jackrabbit 2 - Epic. (PC Only?)
Super Mario World - Don't comment k

(SNES ONLY)
And alot of great platformers.
Great Simulation/Strategy Games:
Sim City 4 - Best Sim City evar! (PC Only? and it's got an Expansion Pack named "Rush Hour" that adds some nice features into the game. Oh also the game is moddable.)
Sims 3 - Sims. Do I has to say moar?
Flight Simulator X - Great Flight Simulator! (PC Only?)
Flight Simulator 2000 - Oh the memories <3 (PC Only?)
The Sims 2 - This was the damn thing while Sims 3 didn't exist. :3
The Sims - This was also the damn thing while Sims 2 and 3 didn't exist. >:3
Constructor - One of the Best. RTS's. Ever. Too bad you can't find it almost anywhere.

(DOS ONLY, it's got a PSX port BUT ONLY IN EUROPE! Use DOSBOX to run this game in Windows XP)
I've made quite a long list so I won't put any moar gamez for a while.