Encyclopedia Metallum: The Metal Archives
Of all the catalog sites I've visited, this website has the most comprehsensive database. It's all metal, and strictly regulated to weed out bands that don't belong or information that is incorrect. If you are looking for information on a metal band, this is the place to go.

The Sound of the Beast: The Complete Headbanging History of Heavy Metal
By far the most informative and comprehensive book I've read on the subject of heavy metal.

Wikipedia
While not the most complete and error free reference, Wikipedia does offer insights and background information on bands that you can't find elsewhere.

A.N.U.S. (American Nihilist Underground Society)
A.N.U.S. approaches heavy metal from a historical and philosophical perspective, and also has an archive of heavy metal bands. Excellent reading, and highly recommended.