The Binding Of Isaac made our Indie Game Of The Year 2011 shortlist, and with good reason. It's an addictive, infinitely replayable and utterly reprehensible 21st centuryhybrid roguelike from the artistic half of Team Meat, and it's as exciting as it is thoroughly stomach-turning. You can currently grab it for £0.59 on Steam, but Value-wise, it's probably best to grab the DLC too for 74p all-in. If you already own it from one of a thousand bundles, the DLC is available separately for £0.29.
Thanks to jaystan @ HUKD!