If you fancy something slightly different, you could try:
- TF2 if you fancy a light-hearted MP game
- Vampire The Masquerade: Bloodlines - a SP first-person RPG game (slightly dated but quality)
- Left4Dead - another therapeutic MP game;
- Bioshock - possibly still £3.50 on Steam
- System Shock 2 (plus gfx update) - somewhat modernised classic.
- HL2 plus Episodes - nuff said.
- CIV IV plus add-ons - if you like addictive strategy games
- Any of the Total War franchise - if you like 'Period' RTSs