Steam has put up three Tomb Raider games for sale through the weekend, and all are a great deal. The remake of the original (Anniversary) is $3.39, while Legend is $5.09 and Underworld $6.79. If you like a good adventure game, either of these are quite good. If you had to choose just one, Underworld is what I'd go for.
http://store.steampowered.com/sale/tomb_raider_anniversary
http://store.steampowered.com/sale/tomb_raider_anniversary