Here're are a few interesting Superhero sourcebooks....
Marvel, DC, and Image characters written up to Marvel RPG rules.
Main page:
http://www.angelfire.com/comics/benriely/index.html
Selected Image heroes (WIldstorm/Image):
http://home.strw.leidenuniv.nl/~vlemming/wldstorm.html
Addl heroes such as G.I. Joe, Cobra, Transformers, etc:
(Note: Gi Joe and Cobra characters and vehicles will be displayed by the year they were introduced, see bottom of page.)
http://www.technohol.com/