Every so often Microsoft holds a bomb ass sale, and this one is no exception. It's been teased for a while now but it goes live today -- over 275 backward compatible (read: Xbox 360 to Xbox One) games are up to 75% off until May 22.
While the landing page is still under construction the sales are live, including all three Dead Space games for $5-$7, and Red Dead Redemption for $12. There's too many classics to name, but out of basically everything I'd highly recommend Fallout: New Vegas, Banjo-Kazooie, Castlevania: Symphony of the Night, Catherine, Dragon Age: Origins, Far Cry 3 Blood Dragon, Ikaruga, Mirror's Edge, Outland, Portal 1, and Spelunky.
There's also some crazy ones in there like de Blob 2 for $3. You can check out the full list here.
Backward compatibility super sale [Major Nelson]