PBS- PBS has dedicated this section of their site to Ken Burns film, Jazz. Aside from being dedicated to the film, the site also contains brief biographies of many influential people in the world of Jazz.
All About Jazz- AllAboutJazz.com is a comprehensive website, with a database consisting of 16,000 musicians. It also has a section featuring many of the key figures of jazz, along with videos and clips of live performances and sessions.
Blank Sheet Music.net- Provides blank scores for composing music.