(RILM, Music Index, etc.)
(Naxos Music Library, Music Online, etc.)see also Free Sound Recordings Resources Online.
Classical Scores Librarysee also Free Scores Online and Free Sheet Music Collections
Reference Tools: Encyclopedias, Dictionaries, etc.
Oxford Music Online, Harvard Dictionary of Music, etc.
MetOpera OnDemand, Naxos Video Library, etc.
[Not seeing what you're looking for? Look for additional library databases here...]
[A guide to Music Research at Yale University]