Mutants and Masterminds
Mutants & Masterminds (abbreviated "M&M" or "MnM") is a superhero RPG by Green Ronin Publishing based on a variant of the d20 System by Wizards of the Coast. The system (now in its second edition) is designed to allow players to create virtually any type of hero or villain desired.
The game mechanics are based on the OGL (Open Gaming License) version of the d20 System. There are, however, a significant number of differences. For example, characters do not have levels or classes, hit points are gone, attacks of opportunity are removed, character creation is via an exclusively points-based system, only the d20 is used, and so on.