8notes.com: Collection and directory of free sheet music pieces and lessons for piano, guitar, violin and other instruments.
Abacci Music: Irish, celtic and world sheet music, midi files and tab.
Anyscore: Mainly commercial sheet music with some free downloads. Includes self-published scores by living composers.
BlankSheetMusic.net: Blank sheet music to print for free, supports multiple keys and formats, guitar and bass tabs too.
Cziffrania: Public domain scores by lesser-known composers. Mostly piano.
Music Sheaf: Features manuscript staff paper in several customizable formats.
NotaViva: Free pieces in PDF format by various Classical composers as well as educational music.
Print Free Staff Paper: Printing free blank staff paper in a variety of formats including ensembles, solo instruments and choral.
Radiohead for the Pianoforte: Free, printable sheet music of some Radiohead songs.
Sibelius Music: Showcase site for the Sibelius plugin 'Scorch' - includes several thousand pay-per-download files and some free users files.