VARIATIONS Prototype: Online Musical Scores from Indiana University
Sibley Music Library at the Eastman School of Music. On the left sidebar, under Collections is a section labeled "Musical Scores." Check out the resources they've digitized from the Sibley Music Library.
Sheet Music and Multi-Media Resources at Harvard If you scroll down on this page, you'll find a lengthy list, including a digital version of the Neue Mozart-Ausgabe! Spend some time with this page of resources. It's simply amazing.
International Music Score Library Project (IMSLP) the public domain music score library. The IMSLP site has been set up - in a wiki format that allows for multiplecontributors.