Frozen Synapse is phenomenal. I bought a multipack ages ago, and remember eyeing it with great interest before release. Finally got around to playing it, and being turn based, it can suck the hours away.
One of the major hooks is that you can simulate the actions you think the enemy will take before you complete your turn. This allows you test a bunch of potential outcomes in advance, and it ratches the tension up tenfold.
There are loads of features built into the client: a chat service (communicate with your opponents without needing Friends list functionality), piles of multiplayer modes, a built in IRC client so you can discuss strategies or find new opponents... There's play-by-email functionality, hotseat (aka pass the keyboard) multiplayer, full coop for the campaign, and I don't think I've even mentioned the DLC yet!
Add all that goodness on top of one of the best soundtracks I've ever heard and a cyberpunk inflected premise and story, and you've got a winner. Immense value for the money. If you have any love for turn-based strategy, be sure to pick it up!