AnonSRD
A ridiculously simple system, invented on 8chan's /tg/ board.
Each character gets six stats. Each stat must be given a modifier from the set [-2, -1, +1, +1, +2, +2]. Stats can be whatever the player wants them to be; there is no set list of attributes or skills to use for stats. Any stat that isn't set for the character is at +0.
All challenges are rolled with 2d6 with the appropriate stat modifier added. On a 3 or less is a crit fail. On 4-6 is a just a failure. On 7-9 is a success, and a 10-12 is a crit success. However, if the player rolls doubles, the result becomes mixed: On a success, there's a hidden drawback and on a failure, there's a silver lining.