International Music Score Library Project (IMSLP) - a virtual library of public domain music scores. It contains the complete works of Johann Sebastian Bach in the Bach-Gesellschaft Ausgabe (1851–99) Besides J.S. Bach's complete public domain works, all public domain works of Ludwig van Beethoven, Johannes Brahms, Joseph Canteloube, Emmanuel Chabrier, Ernest Chausson, Frédéric Chopin, Arcangelo Corelli, Claude Debussy, Vincent d'Indy, Paul Dukas, Gabriel Fauré, Pierre-Octave Ferroud, George Frideric Handel, Jean Huré, Albéric Magnard, Giovanni Pierluigi da Palestrina, Maurice Ravel, Albert Roussel, Erik Satie, Florent Schmitt, Franz Schubert, Robert Schumann, Alexander Scriabin and Jean Sibelius are available as well as a large percentage of Wolfgang Amadeus Mozart, Franz Liszt, and the works of many others as well.
Choral Public Domain Library - Also known as CPDL and ChoralWiki. Free choral and solo vocal sheet music
A useful overview of the Library of Congress classification system for music used at Logan Library