The Mutopia Project offers sheet music editions of classical music for free download. These are based on editions in the public domain, and include works by Bach, Beethoven, Chopin, Handel, Mozart, and many others. The Mutopia archive consists entirely of contributions from people around the world.
If you would like to contribute, there are several ways in which you can help. These include typesetting music that they don’t already have, proofreading music to check for errors, and updating music to a newer version of LilyPond for better quality output and easier updating. Full instructions are given on their website.