Oxford Music Online is a comprehensive site and the best place to start your research. It includes both Grove Music Online and online resources from Oxford University Press.
Updated continuously, Grove Music Online includes overview articles with extensive bibliographies, biographical articles with works lists for composers, performers, and other musical figures, images, musical examples, and links to audio and video examples.
It includes and updates the following print resources:
Oxford University Press titles included are The Oxford Companion to Music and The Oxford Dictionary of Music, 2nd edition.
Easily search across all these resources. Also see the Grove Music Online User's Manual and additional resources for tips on fully using Oxford Music Online.