Mutopia is free, legal sheet music. A longer description from the Scout Project:

Modeling itself after Project Gutenberg, this volunteer project aims
to make copyright-free musical scores available to everyone. Although
copyright on a score expires 70 years after the composer’s death,
music publishers retain copyright on their typeset editions. Thus,
the only legal way to copy this music is to write it out or typeset
it yourself and allow others to make copies. This is precisely what
contributors to Mutopia have done. All music at the site has been
typeset using GNU Lilypond and is available for download as .pdf,
PostScript, or Lilypond .ly files. MIDI files and preview images of
the scores are also provided. Visitors may browse the collection by a
number of options, including composer, instrument, style, and last
updated. Some sections are still somewhat thin, but interested users
will certainly want to pay the site a visit and monitor its future
development.