Some of my favorites:
- Wikipedia Musical Genre: User built info for all genres.
- Ishkur Electronic Music: Guide to electronic music & sample database.
- Discogs.com: Record labels, artists and releases.
- MusicBrainz.org: Music metadata tag creator.
- Weirdomusic.com: Strange, wacky, odd, and just plain bizarre music.
- Mutopia: Free sheet music archives.
- Indie- Friendly Radio: Stations accepting indie artists.
- Musipedia: Search notation & melodies.
- GuitarGeek: Guitar rig database
- mvdbase.com: Music video database.
- Common Content: Copy & re-use audio.
- PD Info: Public domain music.
- Wikiportal Music: Music encyclopedia.
- I Write The Music: Music resources for composers.
- Chordie: Guitar chords, guitar tabs and lyrics.
- Olga Guitar Archive: Tablature & chords.
- The Blues Database: Albums, songs, bands, dicographies.
- Standard MIDI Files on the Net: Links to composers offering MIDI files.
- Unheard Beethoven: Collection of rare works re-created as MIDI files.
- Tinfoil: Early recorded sounds & wax cylinders.
- The Breaks: Database of Hip-Hop samples.
- Project Gutenberg Music: Public domain sheet music, mainly classical.
- Snopes Urban Legends: Tales about the music itself, the people who make it, the physical media on which it is recorded, and the packages that hold it.
- Everything2 Music: Collaborative knowledge base of music & musicians.
- h2g2 music: Guide to life, the universe and everything musical.
- Second Hand Songs: Who performed the original version of a particular song, or who covered that song.
- Connexions: Free scholarly materials, modules & courses.
- Cylinder Preservation & Digitization Project: Over 5000 recordings for download.
- Virtual Instrument Museum: Multimedia collection of musical instruments.
- Microtonal & Just Intonation Theory: Encyclopedia of microtonal music theory.
Complete list: Dave’s Imaginary Sound Space