GNU LilyPond is Open Source sheet music typesetting software for Linux and Windows. The typesetting language is straightforward, though it might intimidate novice users. If that's too much for you, there are several other ways to make LilyPond files, the most attractive option being RoseGarden, a full sequencer, audio editor and notation package. You can even try to translate a MIDI file into LilyPond. This mildly interesting mailing list message critiques LilyPad and the general notion of music notation software.

The Mutopia Project is one of several public domain sheet music projects, a sort of Project Gutenberg for sheet music. They offer PDF (Adobe Acrobat) files, PostScript files, and even LilyPond typesetting files.

The Choral Public Domain Library (CPDL) has downloadable sheet music for public domain choral music.

Project Gutenberg has a sheet music section of its own. Adopt a Beethoven Quartet!