Anything thats worth playing.
I got FTL (Faster Than Light) for $4.99 on sale a few weeks ago. I recommend it :)
super hexagon is pretty fun... in a smash your computer kind of way
it's in the current humble bundle, but i think its $3 or so on its own on steam
Also recommending FTL: Faster Than Light, but it's only $5 when on sale
Lone Survivor looks like another really good game from everything I've seen if you like survival horror
And I managed to get Borderland 1 for $5 during the Winter sale. Great purchase that was
Any of the classic Tomb Raider games for nostalgia.
humble bundle, $5 USD get 4 games, most that are quite fun. Choose how much you want to donate to charity and etc.