Medieval Music:
La Trobe University Library Medieval Music Database
Modern scores and facsimiles of original manuscripts; although no longer maintained, still provides considerable important content
Thesaurus Musicarum Latinarum
"...free access to and make searchable every known Latin text on music from the late antiquity to the seventeenth century, in multiple editions and in transcriptions from original sources."
Abbaye de Solesmes
From the abbey renowned for spearheading the revival of Gregorian chant in modern times
Renaissance Music:
RELICS Renaissance Liturgical Imprints: A Census
Hill Museum & Manuscript Library
a vast database of manuscript images, including music, from St. John's University and St. John's Abbey in Collegeville, MN; includes the Renaissance and much more
Classic and Romantic Music:
The Ira F. Brilliant Center for Beethoven Studies from San Jose State University
Ethnomusicology:
Ethnomusicology
Ethnomusicology, Folk Music, and World Music an important ethnomusicology portal from the University of Washington
British Forum for Ethnomusicology
"a UK society devoted to the study of music and dance from all parts of the world"
Ethnomusicology
the journal; a website providing information about it with sample full-text articles
Other:
Center for the History of Music Theory and Literature
from Indiana University; includes Doctoral Dissertations in Musicology and several research sites
Includes Thesaurus Musicarum Latinarum