Here are some other noteworthy websites dedicated to the Marvel D100 system:
http://rivendell.fortunecity.com/battlespire/85/homepage.htm - The creators of this website must really know their superhero RPGs. They have listed conversion charts from most major superhero RPG systems into Marvel D100. Lots of other information is presented, including information originally available only from Dragon Magazine in the Marvel-Phile. Very well done.
http://webspace.webring.com/people/jt/the_black_condor/ - This website has a unique focus. It is purely devoted to charting the alternate realities in (and out) of the Marvel Multiverse. World write-ups and character stats are provided. So when the players, say, get their hands on Doctor Doom's Time Machine or travel to other realities, this website is there to help out the Judge in need!
http://www.angelfire.com/planet/heroicus/files/stats-2.html provides character write-ups and stats from dozens of non-Marvel properties, formatted for the D100 system.
http://marvel.wikia.com/Main_Page The Marvel Database is always a useful tool for clarifying continuity or character questions.
So there you have it! A basic beginner's guide to Marvel D100 on the web!