First up is Classic Marvel Forever at http://www.classicmarvelforever.com/cms/. This website is quite impressive. A good deal of the material written here is actually written by the same people the wrote parts of the original game, notably the Super Heroes Handbook. You have Addendas (which are like unofficial erratas) and rules clarifications that are very helpful.
There are a ton of house rules listed on this site.There's information about technology in the Marvel Universe, D20 conversions and much, much more.
One of the favorite things of fans of a system, especially a dead system, is to keep it current. You'll find tons of updates for characters that were created after the classic Marvel RPG ceased publication such as the Sentry or Echo. You'll also find character updates for old characters like Iron Man in his Extremis armor or Spidey in his Marvel Civil War costume.
In addition to this material, there's plenty of non-Marvel material using the classic Marvel resolution system. Like the D100 table but you're a fan of DC? No problem - they've got stats for Superman, Batman and the lot at Classic Marvel Forever. The same is true for Dark Horse and Image comics characters as well. You could do your own inter-company crossover using the information provided at Classic Marvel Forever! There are even stats for other properties such as Akira and Final Fantasy.
Classic Marvel Forever is one of the premier websites for information regarding the D100 Marvel RPG system.