Petrucci Music Library (IMSLP) Canadian Web site sponsored by the International Music Score Library Project and dedicated to creating "a virtual library containing all public domain music scores, as well as scores from composers who are willing to share their music with the world without charge."
Opera and Ballet Primary Sources (OBPS) A growing collection of primary sources useful for opera and/or ballet research; nearly 10,000 scanned libretti and scores made available online. The OBPS index also includes many scores, sets of parts, mise en scène, and other useful primary sources related to opera, ballet, and 19th-century vaudeville. Maintained by Brigham Young University and the City Archives of Brussels.
Sheet Music Consortium Searchable database of images of sheet music from collections throughout the United States. A project of "a group of libraries working toward the goal of building an open collection of digitized sheet music" maintained by the University of California, Los Angeles.
Online Resources for Music Scholars An extensive, searchable database of websites, catalogs, databases, and other scholarly resources in music maintained by the librarians at Harvard University libraries. Note: Many of the entries link to Harvard-only sites, but many others are open access.