Tower Defence, Action-RPG hybrid Dungeon Defenders is only $10 or 800 Microsoft Points this week. If you haven't tried the game, do something nice for yourself and download the demo. There is literally tons of hours of gameplay to be enjoyed and even more if you've got a few friends willing to hop online with you.
I haven't played the game much on either the PC or Xbox 360, but owning a copy on both platforms is a testament to how much I've enjoyed what little I've played.
The 33% discount lasts a week on Xbox Live, so be sure to try the game and buy it before the end of the sale on January 17th.