National Library of Australia's Sheet Music Collection
Juilliard Manuscript Collection
ViFaMusik: Virtual Library of Musicology
Internet Culturale: cataloghi e collezioni digitali
(1) If you are looking for a particular source, search in the "Biblioteca digitale," not in the listings under "Collezioni digitali."
(2) The database of "Cataloghi" (which incorporates the Italian national library catalogue, SBN) contains some links to digitized sources not included in the "Biblioteca digitale," e.g. the great libretto collection at the Biblioteca Nazionale Braidense in Milan.
Many music resources are not digitised yet.
Monash has the following
Australian Sheet Music Archive
Popular Sheet Music Collection
British Library music collection
European musical instruments on prints and drawings
Mediaeval & Renaissance manuscripts at Trinity College, Dublin
Regole, passaggi di musica, madrigali, e motetti passeggiati / di Gio. Battista Bovicelli d'Assisi
For more details on the microform collection see the following guide: