Speaking from my limited experience, MMPRGs are usually pretty non-addictive.
I'm more of a console gamer, but The Sims 2 or Civilization IV is pretty fun.
If your looking for something more casual, try Tetris.
Quinn (free) is a good Tetris clone, but I perfer
Tetris Zone ($9.95).
Civilization 4. Extremely subject to 'one more turn' syndrome...
I know what you mean. Sometimes I'll start playing at 9:00PM and then before I realize it, it's 1:30AM.
I literally have to force myself to get off. Maybe that's why they have a built in clock.