Academic Search Premier is a large database that covers more than 8,000 journals, 85% of which are peer-reviewed and over 55% are full text, and full text for nearly 3,900 peer-reviewed titles. It covers a wide variety of disciplines.
JSTOR's Arts & Sciences Collections I-IV includes core journals in economics, history, political science, and sociology, as well as in other key fields in the humanities and social sciences, including Asian, African, Latin American, Middle Eastern, and Slavic studies, archaeology, classics, languages and literature, folklore, music, film studies, performing arts, religion, art and architecture, feminist & women's studies, business, education, law, psychology and public policy and administration.
Some fascinating links sorted into music by type; institutions, organizations and labels; music theory, therapy, and education; women in music; and others. Not checked very recently, as some of the links are dead, but an interesting collection.
A wide and varied wiki of music scores and MIDI recordings of music in the public domain (after the period of copyright has expired) of Canada, the US, and the EU. While the point at which works enter the public domain is different for Canada, the US and the EU, there is enough overlap that there's plenty to explore, by composer, performer, or work.
A commercial site that offers some value as a database of song and album information. Useful when assembling citation information (or if you'd like to know what albums can be thought of as, say, "Autumnal").
Housed at the Internet Archive, the Ethnomusicology Archive from UCLA contains more than 150,000 audio, video, print and photographic items about musical traditions around the world, going back as far as 1961.
Musical works are cited in different ways depending on their medium. Sound recordings are different from sheet music which is different from .mp3 files, so it's important to be specific when you cite a work you have used.
MLA (Purdue OWL; includes sound recordings, spoken word albums, and digital files)