I am suprised that no one has said Heroes of Might and Magic 3. That game is actually what started my eventual slide into gaming (and by slide I mean more like avalanche). The addictiveness of that game has caused me many, many sleepness nights. I can not tell you how many times I have tried to beat a map on the highest difficulty setting. I can't ever get tired of that game.

On another note, I also can't get tired of playing Burn the Rope, a flash game on Kongregate, only because it makes me laugh everytime I do.