Killing Floor started years ago as a mod of Unreal Tournament 2004, but eventually was made into its own game and released for PC in 2009. The game was eventually upgraded once again and released by Tripwire Interactive as Killing Floor: Calamity exclusively on the Ouya back in December. For those that missed out back then, there is an excellent sale going on right now for it.
For an indeterminate amount of time, you can get Killing Floor: Calamity on your Ouya for 70% off the usual price. This means you will only be paying $4.50 for this exciting top-down shooter. You can play by yourself or with a friend in two-player co-op. If you are interested, make sure to act fast as this sale has no set end date.