I'm only just now getting around to Frozen Synapse, which was released back in May and about which a bunch of people I know have had only nice things to say. So far, it's like some glorious combination of X-Com and Tron with maybe a touch of Rainbow Six and Ubisoft's fantastic 3DS game Ghost Recon: Shadow Wars. (Which is still my favorite 3DS game—Crecente agrees.)
Frozen Synapse's developers, Mode 7, are now offering a free demo on their website, and this weekend, the full game is 50% off on Steam. That means that for $12.99, you'll actually get two copies of the game, one for you and one to send to whichever friend you'd most like to cleverly ambush and kill, over and over again.