There are many good things about Call of Duty 4. This is the best. Throughout the game, you collect various pickups, in the guise of laptops containing enemy intel. Collect enough and you can unlock hugely silly cheat modes, such as grenades becoming cluster bombs or the colours being inverted (which is really not pretty).
The real prize is ragtime mode, which turns everything sepia, speeds the game up by 20%, mutes all the sound and then adds a frantic 1920s plinky-plonk piano soundtrack. Modern Warfare becomes Buster Keaton movie:
http://www.rockpapershotgun.com/?p=549
