Jamey Aebersold Jazz, Inc.: A good source of published transcription books.
Hal Leonard: Another publishing company that offers transcription books.
Active Musician: Contains transcription books as well as sheet music.
A Passion For Jazz: Jazz history, festivals, webcasts, piano chords, scales, online lessons and music teacher locator.
The Music Directory: A collection of links to music related websites and newsgroups.
Reynolds Pro Music: A site with with transcribed solos by John Reynolds.