The International Music Scores Library Project (IMSLP) aims to catalog all the free, public domain music scores available online in one simple, searchable interface. The Project was started in 2006 by Edward Guo, a graduate of the New England Conservatory of Music in Boston and currently a doctoral student at Harvard Law School.
IMSLP, commonly called the Petrucci Library after the Italian sheet music printer Ottaviano Petrucci, has over 210,000 scores and 20,000 recordings. The Library is a wiki searchable by composer, nationality, genre, and more. But the Library is more than a collection of music: the IMSLP Journal and Forums are digital meeting places for musicians and music lovers to share ideas and collaborate on projects.
IMSLP works hard to follow all copyright regulations, as The New York Times reported last year, though this is difficult given that different countries have different rules. Volunteers help check for copyright violations. Considering signing up to become an IMSLP contributor yourself and help keep the project going!