Masks appears to be a completely free, pretty rules-light system.
http://www.magpiegames.com/masks/
I haven't looked too hard at it yet, as I just discovered it while reading this thread, but it looks promising. I would be up for a game in this system if anyone was running it.
Another option would be the old Marvel Super Heroes RPG. Also available for free legal download. I don't have a link handy right now, though. It's a little less stratified than Masks appears to be, and a little more rules heavy, but it's a good system.