The Humble Store is running a spring sale from May 6th through May 21st, with some pretty good savings on a number of games, including some recent titles like Thief and South Park : The Stick of Truth.
10% of all Humble Stores go to charity, and their weekly and roughly monthly pay-what-you-want deals are usually pretty good for picking up games and occasionally other items (right now the main Humble Bundle is a set of digital comic books).
Most of the games net you Steam keys, so you will need a Steam account to redeem them.