Free Systems

Men & Supermen
A game distributed under the GNU Free Documentation License.
http://www.hoboes.com/html/RPG/BrandX/

Panels
Tabletop game which paints the world of superheroes in broad strokes. Numerically light, almost free form rules.
http://www.geocities.com/Area51/Quasar/9229/supes/

Powergame Home Page
Powergame is a free superhero RPG which has been in development since 1994. It is designed to cater for all levels of superheroic campaigns within a simple rules framework.
http://www.kotiposti.net/trmika/powergame/

Risus Supers Version 1.1
Provides simple game mechanics to simulate superheroes who no one takes seriously. Includes character cliches and rules for common contests.
http://www.angelfire.com/tx2/webgamer/supers.html

Rumble
A simple and fast paced system with a large list of super powers.
http://kevan.org/rumble.cgi?rules

Super Heroes and Heroines
Designed to portray superheroes who have personal lives.
http://members.tripod.com/~TrevorDoom/