Matters on what game type you want to mod. For FPS there's Half Life 2, UT2k4, Doom 3, and a wack load of others. If you would rather try RPG you can look into Dungeon Siege, Neverwinter Nights or Morrowind. Dungeon Siege 2 will be out seen, hopefully it's decent for modding. Also, when The Elder Scrolls 4: Oblivion comes…