http://www.geocities.com/marvel_villains/ A great site. let's face it, the villains are always the coolest characters. Nicely arranged, lot's of info
http://www.flash.net/~jeanneb/g.html - A site, dedicated to the second-stringers. From Devil Slayer to Machine Man, full of those obscure characters that everyone loves.
http://www.chronologyproject.com/ - The Marvel chronology project. No flashy graphics or complicated guff. Just fact. And good fact at that.
http://www.users.xeroanime.com/~handbook/ - The Unofficial Handbook to the Marvel Universe. Possibly my favourite site. Information on just about everyone who's ever appeared in a Marvel comic.
http://www.marvunapp.com/Appendix/ - The appendix to the Unnoficial Handbook to the MArvel Universe. Alternative histories, futures, different dimensions, what if stories, massively obscure characters... great site.
http://hometown.aol.com/drg4 - Guide to the early 90's Marvel cartoons. Hulk, Spidey, X-Men, Fantastic Four, Iron Man and Silver Surfer. Great sitre for fans of the cartoons.
http://www.x-women.org/ - Often neglected. The lovely ladies of the X-Men and their allies.
http://www.marveldirectory.com/ - Much like the Unnoficial Handbook, this is a massive and comprihensive guide to Marvel characters.
http://home.att.net/~the_avengers/ - Cool Avengers site. 'Nuff said.
http://www.geocities.com/Hollywood/2855/ - Site devoted to the women of Marvel Comics.
http://wolverine.x-knights.com/wolverine_main.html - Wolvie has approximately 500,000 pages on the 'net. This is one of the best.