Mythender is actually free online! Masks is not, unfortunately, but it's only like 14 bucks.

Mutants and Masterminds goes in a great direction, with superpowers that are relatively abstract and effects-based, which keeps them pretty clear and concise. Not a huge fan of how it works in practice, though. Tying everything to the d20 is a bit of a weakness of that system. ICONS, made by the same creator, takes this a bit further and makes some neat improvements. I've never gotten a chance to play it, but it seems pretty neat.