Gamersgate are doing a different deal every day until Sunday on Square Enix games.
This is actually a fantastic bunch of games if you don't have them. Deus Ex is a little older, but Just Cuase 2 will be a steal, and registers on Steam.
Here's the details.
Thursday: Deus Ex: Game of the Year Edition (£2.25) & Deus Ex: Invisible War (£1.25)- 75%
Friday: Lara Croft and the Guardian of Light (£2.49)- 75%
Saturday: Just Cause (£2.50) & Just Cause 2 (£3.49)- 75%
Sunday: Batman: Arkham Asylum Game of the Year Edition (£2.99) - 80%