I wrote (read: stole most mechanics of) a simple Superheroes game whereby the players chose their superpower, and powered it through use of XP. They basically had to fill in a little thing on the sheet, "For 0xp, I can..., For 1xp I can..." etc up to 5xp. So for example, one character had the ability to create doorknobs for free, and then could attach them to to something and pull them open like a door, for XP depending on the size of the door and the thickness of the thing the doorknob was attached to. Since we used candy for xp (my favourite was fruit tingles) people would seek to spend their XP pretty quickly (you got XP by playing your character or making the story push forward).
It seemed a lot to do with setting for my guys. People who came up with powers that tried to game the system found that they weren't having as much fun as those whose powers were more light-hearted and fluffy (eg Disco Inferno, whose powers were something like, For 0xp I can make any musical device play funk; for 1xp I can make my gun shoot louder than anyone else's gun... up to for 5xp I can force all people in the area to take part in a choreographed dance scene like a musical). Also, since each character had to pick a secret shame/flaw for their character, which was basically designed for me to use against the players if they were being dicks, those with cheesy powers just found that their secret shame came up constantly, whereas those who chose quite nasty flaws to begin with had a good chance of being able to mitigate them by their own actions.
Was a great, fun system to play for a few laughs, sort of like an icebreaker game to get people into the idea of roleplaying, before thrusting a more complex system at them.
