RISM (Répertoire internationale des sources musicales)
RISM's catalog is a finding aid for music manuscripts and prints; the online catalog includes Series A/II (Music Manuscripts after 1600) as well as a list of library sigla. Series A/I and Series B (which are available in print only) can be found in the Orwig Music Library.
Music Division Collections at the New York Public Library
"The Music Division is one of the world's preeminent music collections--documenting the art of music in all its diversity - classical and opera as well as the whole spectrum of popular music including spirituals, ragtime, jazz, musical theater, film, rock and world music. While the division contains many scores and manuscripts from centuries past, its curatorial mandate is an activist one, placing major emphasis on capturing the creative output of contemporary composers."
DIAMM: Digital Archive of Medieval Music
Since its founding in 1998, "the purpose of the Digital Image Archive of Medieval Music (DIAMM) [has been] to obtain and archive digital images of European sources of medieval polyphonic music, captured directly from the original document."
RId'IM: Répertoire International d'Iconograpie Musicale (International Repertoire of Musical Iconography)
The RId'IM database contains descriptions of visual objects that depict music-making, musical instruments, and similar related objects. Where possible, there are links to online images of the artwork.
University of Washington Ethnomusicology Musical Instrument Collection
A database featuring the 400 musical instruments from around the world held at the University of Washington.