8notes.com
   Free sheet music pieces for piano, guitar, violin and other instruments and free lessons. Directory of free sheet music pieces on the internet.
   http://www.8notes.com/

Abacci Music
   Irish, celtic and world sheet music, midi files and tab.
   http://www.abacci.com/music/default.asp

Anyscore
   Mainly commercial sheet music with some free downloads. Includes self-published scores by living composers.
   http://www.anyscore.com

BlankSheetMusic.net
   Blank sheet music to print for free, supports multiple keys and formats, guitar and bass tabs too.
   http://www.blanksheetmusic.net

Music Sheaf
   Features manuscript staff paper in several customizable formats.
   http://www.musicsheaf.com

NotaViva
   Free pieces in PDF format by various Classical composers as well as educational music.
   http://www.notaviva.com

Online Music Scores
   Links to scores which may be downloaded free of charge, collected by Howard J. Callon, music professor at McGill University in Seattle, Washington.
   http://plato.acadiau.ca/courses/musi/callon/2273/scores.htm

Sibelius Music
   Showcase site for the Sibelius plugin 'Scorch' - includes several thousand pay-per-download files and some free users files.
   http://www.sibeliusmusic.com