8Notes.com
http://www.8notes.com/
Free sheet music and sound files. Browsable and searchable.

BigBands Database Plus
http://nfo.net/
Jazz and Swing history, biographical information, and sound files galore.

Classical Music Pages
http://w3.rz-berlin.mpg.de/cmp/Welcome.html
Historical and biographical information in the field of Classical music.

Music Acoustics
http://www.phys.unsw.edu.au/music/
Information about instrumental and voice acoustics.

Music History 102
http://www.ipl.org/div/mushist/
The Internet Public Library offers audio files and textual information about Western classical music composers from the Middle Ages to the present.