Based around a free(!) ruleset, Four Color System from Seraphim Guard Games, which is not only free to own, but available to other publishers wishing to write for it.

A major line of product comes from Highmoon Games, a series of 'Heroic Moments' - a single action scene. The series is designed to be dropped seamlessly into any existing superhero campaign. No prep: just a set-up, some bystanders & a villain for your heroes to clobber.

Vigilance Press has also released materials for the 4 Coloured System.