http://www.pcgamer.com/sanctum-2-is-free-in-the-humble-store/

 Sanctum 2, the FPS-tower defense hybird that I think is actually quite good, is free for the next two days as part of the Humble Store Fall Sale. You know the drill by now, right? Go here, add to cart, check out, have a nice day. Still got an urge to spend some money? They can help with that, too. 

Most of the Humble prices are the same as what's being offered in the Steam Autumn Sale (Last Inua is slightly cheaper), but a percentage of Humble Store purchases go to support charities, which is solid, and hey, they're giving you a free game. Only until November 25, however, so get on it if you want it.