I found out about this via o’reilly radar’s blog..

Musopen is an online music library of copyright free (public domain) music. We want to give the world access to music without the legal hassles so common today. There is a great deal of music that has expired copyrights, but almost no recordings of this music is in the public domain. We aim to record or obtain recordings that have no copyrights so that our visitors may listen, re-use, or in any way enjoy music. Put simply, our mission is to set music free.

The amount of music available is not too bad (and constantly improving):

Albéniz, Isaac Albinoni, Tomaso Anderson, James Bach, Johann Sebastian Beethoven, Ludwig van Berlin, Irving Berlioz, Hector Bettinelli, Bruno Borodin, Alexander Brahms, Johannes Brubeck, Dave Bruch, Max Bryars, Gavin Chopin, Frédéric Clementi, Muzio Copland, Aaron Croce, Giovanni Debussy, Claude Dunn, Aaron Dupre, Marcel Dvořák, Antonín Elgar, Edward Franck, Cesar Glazunov, Alexander Gluck, Adolph Granados, Enrique Grieg, Edvard Handel, George Frideric Haydn, Joseph Holst, Gustav Jr, Johann Strauss Laforge, Agathe Lavallee, Calixa Liszt, Franz Lotti, Antonio Mardirosian, Haig Mendelssohn, Felix Mendelssohn, Felix Miguez, Leopoldo Monteverdi, Claudio Mozart, Wolfgang Amadeus Mussorgsky, Modest Part, Arvo Pascucci, Massimiliano pergolesi, g.b. Perotinus, Magister Premrl, Stanko Prokofiev, Sergei Purcell, Henry Rachmaninoff, Sergei Ravel, Maurice Richards, Goff Rimsky-Korsakov, Nikolai Root, George F. Rossini, Gioachino Russell, Alexander Saint-Saëns, Camille Satie, Erik Scarlatti, Domenico etc….

Also, they have a growing collection of sheet music, eg: