Cheat Engine - Cheat Engine is an open source memory scanner/hex editor/debugger for Windows.
WeMod - Personalize your gaming experience Thousands of free mods and trainers for your favorite single-player PC games — all in one place.
scanmem & GameConqueror - GameConqueror is a GUI for scanmem.
ArtMoney - Forget about cheat codes, no more cheat codes, Universal Memory Editor.
GameCopyWorld - An index for Game Backup Instructions, No CD Patches, and Game Trainers
GameGuardian - Play games your way! “GameGuardian” is a game hack/alteration tool.