Infernal Teddy said:
I dunno. M&M is d20, right? Can d20 really handle Superheros well?
M&M uses a d20-like resolution system, but most of the game engine is closer to
Champions - point-based system to buy relatively generic powers which are then customized to fit a concept. That model has worked pretty well for superheroes for 25 years, and M&M does it well.
Other decent choices:
HeroQuest works very well for supers, and converting the rules to that genre is trivial. If you want a game where Spider-Man gets his butt kicked by the Green Goblin on first fight, but then the Goblin goes after Spidey's family and friends and that drives Spider-Man to extra heights in order to squeak out a win and save the day... HQ does this beautifully.
Capes! is an indy superhero game with a radically different approach to RPGing, in effect there is no GM (or more accurately, everybody shares the GM duties). Some like this approach, some hate it, but those who like it seem to like it a lot.
Silver Age Sentinels and The Authority both use the Tri-Stat dX system, which is
freely available. SAS was considered the main contender after Hero back before M&M appeared, and even though it's gone into the doldrums the system is still solid.
Another freebie is
Powergame, which is a simple, fast-playing system that still lets players roll lots of dice (I confess that one of the reasons I love Hero is because you get to roll big handfulls of dice). IME, it plays a lot like the old FASERIP Marvel game, fast and furious. Actually, you can also find that Marvel game on the web in various places, but I'm kind of fuzzy on the legality.
Fudge has a couple of good supers add-ons. The 10th anniversary book has an interesting supers system that I've been tinkering with, it's got a lot of really cool points that handle the genre well. Or for a simpler approach, there's a conversion of the FASERIP keyword system to Fudge
on the web. It was written by some guy named Steve Kenson, who knows a thing or two about supers.
There's lots of other games out there, the supers genre attracts a lot of attention despite the fact it's a pretty small niche in the RPG market. These are just the ones I've seen work well, many other games also have their fans.