Mentioned already on the MK thread, but playing Mortal Kombat. To summarise, fucking brilliant. Love it so far.
Also playing Terraria, an indie game on Steam. One of the best games I've played in a while. It's been described as a 2D Minecraft, and although I've never played Minecraft, I can see why. Played it a lot more since first mentioning it on another thread, and it's a lot of fun, and completely satisfying. No real goals other than to explore and create. I'd really like to try out the multi-player side of Terraria, so if anyone decides to try it (only £5.99!), let me know