Petrucci Music Library (IMSLP) The International Music Score Library Project 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. Also includes many scores, sets of parts, mise en scène, and other useful primary sources related to opera, ballet, and 19th-century vaudeville.
Sheet Music Consortium Searchable database of images of sheet music from collections throughout the United States.
Online Scores / Sheet Music Extensive list maintained by Duke University that includes online collections such as DIAMM (Digital Image Archive of Medieval Music) and 19th Century American Sheet Music.
Hathi Book Trust A partnership among major research institutions and libraries devoted to providing digitized books, scores, and other texts found in public domain or made openly accessible by copyright holders. To search for scores, use the "Advanced Full Text Search" option, and limit the original format search to "Music Score."