New Grove dictionary of music and musicians Key English-language encyclopedia of music (via Oxford Music Online), including The new Grove dictionary of opera / … of jazz.
Naxos Music Library (NML) Classical music audio streaming database with over 125,000 CDs of almost all major classic labels. After use, please finish the session by clicking on "Log-Out"!
RILM Key bibliography for dependent literature (journal articles, etc.) and secondary literature on music (from 1967)
MGG Online Central German-language encyclopaedia of music (includes contents of MGG2 plus updates).
RIPM List of articles from more than 120 music journals from 1800 to c. 1950, often with full text access (RIPM Online Archive)
RISM List of music manuscripts from 1600 to c. 1850, with incipits and location information