Mutopia Project

The Mutopia Project is a volunteer-run effort to create a library of free content sheet music, in a way similar to Project Gutenberg's library of public domain books. It started in 2000.
The music is reproduced from old scores that are in the public domain. New scores are digitally typeset with GNU LilyPond and distributed in the following formats:
Currently, there are more than 2,000 pieces of music available, more than half of which are pieces for piano. There are also many pieces for voice, and various other musical instruments. The Mutopia Project home page has a list of links to the most recently added pieces.