International Music Score Library Project. (IMSLP/Petrucci Music Library)
The IMSLP was created to provide public-domain music scores free of charge to anyone with Internet access. It includes music scores in PDF versions; audio files of some pieces of music, with downloadable MP3 files; and music-related forums and a blog. Browse by composer; browse recordings by composer; and search by melody, instrumentation, or genre.
Library of Congress. Performing Arts Encyclopedia.
Digitized collections at the Library of Congress covering the performing arts.