Currently included Websites:

The Unofficial Appendix to the Marvel Universe.
Easily the best site for Marvel bios on the web, though it focuses on the obscure. Each entry is annotated so you can check their facts, which greatly reduces the "fan fiction" factor of people wanting to do "cool" things like writing themselves into Wolverine's backstory. The site's owner, Jeff Christiansen, is the lead writer for Marvel's Official Handbooks.
After you do one search, you can also "refine" your search to look at only this site, if you like.

The Master List at the Appendix to the Marvel Universe.
Located on the same site as the Unofficial Appendix, the Master List is nothing more than an alphabetical list of characters listing every appearance of virtually every character in the Marvel Universe. "Refining" your search to the Appendix does not include the Master List.

Marvel Comics: the Marvel Universe
This includes only the moderated wiki "Marvel Universe" portion of the Marvel website, and ignores the bulletin boards, forthcoming comics blurbs, etcetera.

Again, only character biographies are included and the rest of the site is excluded. As you might guess from the name, a wide variety of Spider-Man characters get bios here. Jonathan Couper, the site owner, worked briefly for Marvel on the Spider-Man Encyclopedia and their 2005 Spider-Man Handbook.

Marvel Directory.
A compilation of biographies which appear to be mostly information hijacked from the Official Handbooks (can anyone say "copyright violation"?). Still, as such, it appears to be very reliable.

Marvel Guide.
Heavy on "statistics," light on oratory. One of the few places that provides issue citations, so one of the better sites by default!

Uncanny X-Men.
Semi-moderated information on the sub-universe of the X-Men.

A Guide To Marvel's Golden Age Characters.
Short and unannotated bios of Marvel's Golden-Agers. From that site, this search engine includes only characters linked from the Golden Age and Pre-FF 1 sites.

Websites I personally recommend but don't include here for various reasons:

Marvel Chronology Project
Unofficial Handbook of Marvel Comics Creators

This site has no affiliation with Marvel Comics, and the selection of sites herein should not be read as an endorsement nor as an acknowledgement of those sites by Marvel.