Mafia 2 is on deal right now (it’s either $7.49 or £4.99), the game uses Steamworks so it activates on Steam nicely. I really recommend this one, if you like open world games and great story line you won’t regret this one. Check it out here. Unfortunately to me it says that I can’t purchase it outside UK.
RollerCoaster Tycoon 3 Platinum 3 is also on deal with a price a little over £3. You can grab it here. I have no idea if it works on Steam.