[mckerney]

1. Dota 2 (Nearly 900 hours more than anything else, so unlikely to change soon)
2. Left 4 Dead 2
3. Dragon Age: Origins
4. Mass Effect 2
5. Skyrim
6. Team Fortress 2
7. Terraria
8. Civ IV
9. Mass Effect
10. Fallout: New Vegas (52 hours)

Likely to at least pass where New Vegas is by end of next year:
Hero Academy
XCOM
Civ V
Borderlands 2
Saints Row 3

EDIT: New Vegas is actually slightly ahead of L4D.