Sibley Music Library Musical Scores
You will need to create an account to use this database. The University of Rochester's Sibley Music Library is the largest academic music library in North America. It has been engaged in an NEH project to digitize its public-domain music scores, emphasizing scores not widely held by other libraries and not digitized elsewhere. To date the library has digitized over 22,000 publications, including, in addition to scores, some books and a variety of handbooks, guides, and theses.
More resources are constantly being added. Other websites offer digitized scores, but this site is unique because of its size and ease of use and because its content is free.