Cheat Engine - Cheat Engine is an open source memory scanner/hex editor/debugger for Windows.
GameCopyWorld - An index for Game Backup Instructions, No CD Patches, and Game Trainers
scanmem & GameConqueror - GameConqueror is a GUI for scanmem.
Fling Trainer - Fling Trainer gives you the opportunity to download PC Game Cheats and Mods for many games without asking for any payment in return.
WeMod - Personalize your gaming experience Thousands of free mods and trainers for your favorite single-player PC games — all in one place.
ArtMoney - Forget about cheat codes, no more cheat codes, Universal Memory Editor.