Allmusic.com
Provides comprehensive music information including reviews and biographies.
Aria Database
Search the database of over 1000 operatic arias by title, composer, or character name.
CPDL (Choral Public Domain Library
A free sheet music website specializing in choral music. The goal of CPDL is to host a large collection of music scores and sound files which can be freely downloaded and used. Most of the scores on CPDL are modern editions based on older works whose copyright has lapsed. Some scores are newly composed and offered for download by the composer.
Essentials of Music
Music theory and history basics, as well as a glossary of music terms.
IMSLP.org
A virtual library of public domain music scores based on the wiki principle. Users can exchange musical ideas through the sire, submit their own compositions, or listen to other people's compositions. IMSLP includes many historical editions of both instrumental and vocal works, as well as contemporary works by American and Northern European living composers.
Musicals.Net
Synopses and song lists for many popular musicals.
Musicals 101
A guide to basic resources on musical theatre, television and film.
www.8notes.com
Offers free sheet music, lessons, and tools for musicians who play. Also includes biographies and videos.
