Bassist Andrew Harmon (of That’s My Cone! fame) sent me links to two extremely useful repositories for freely downloadable scores. The first (which I have used for research purposes before–extremely useful) is the International Music Score Library Project. Founded in 2006, this project is an attempt to compile all public domain scores into a virtual library. With over 11,000 scores (and with more added every day), this is a amazing resource for musicians. Check it out!
The next project is the Neue Mozart-Ausgabe: Digitized Version. Although the interface is significantly less user-friendly than the IMSLP project, this provides a complete and freely accessible and downloadable catalog of the same source material used in the Bärenreiter edition of Mozart’s works.
The NMA Mozart project places restrictions on the use of material on the site, making you click through a license agreement that restricts you to personal use (not public performance use). No such restrictions exist on the 11,000+ works in the IMSLP (thanks to the public domain status of these works), and the interface is very similar to Wikipedia. You are encouraged to edit information and upload new material (that is in the public domain), just like other wiki sites. Truly an amazing resource for musicians worldwide!