Oxford Music Online [24/7] - Huge music resource from Oxford University Press, incorporating the New Grove Dictionaries of Music & Musicians, Jazz, and Opera; and Colin Larkin's Encyclopedia of Popular Music.
Naxos Music Library [24/7] - Listen to over 90,000 CDs, including the whole Naxos catalogue. Classical, jazz, nostalgia, world and pop. Read sleeve notes, glossaries and libretti, exam texts and fun stuff for kids.
Oxford Reference Online [24/7] - The music section from Oxford's great online reference resource.
Music Reference Online [24/7] - Music reference resources from Alexander Street Press, including Classical Music Reference Library, Garland Encyclopedia of World Music, African American Music Reference, and Classical Scores Library. [Can't get in?]
Westminster Music Library - Music scores and a wealth of information available to the public. You can search the library's holdings of sheet music using the Song Index or the Catalogue of Orchestral Sets
International Index to Music Periodicals [24/7] - Indexing and abstracts for several music journals and magazines plus some full-text articles.
Making Music - Find a community music group to join, or a concert to attend.
allmusic - Search for music by artists, albums, songs, styles or labels.

Classical Net - Index of composers and works. Basic repertoire of recordings, reviews, articles and links to other classical music sites.
MusicRoom - Commercial site claiming to have "the world's best selection of sheet music, books about music, tuition books, music software, and instruments & accessories".
The Gramophone - Complete searchable and browseable archive of The Gramophone magazine since its start in 1923. There is a time lag before the current edition is available to non-subscribers.
Operabase - Reviews and guides to performances, guide to composers and works, and links to other opera resources.

Garland Encyclopedia of World Music [24/7] - Comprehensive encyclopedia devoted to musical traditions from around the World.
Contemplator - Words and music (tunes on midi files) for 100s of folk songs from England, Scotland, Ireland, Wales and America.
The Full English - The world’s largest free digital archive of English folk songs, tunes, dances and customs, containing over 58,000 items from the country’s most important early 20th century folk music collections.
English folk and traditional music - A very long list of useful weblinks relating to English folk music.
National Jazz Archive - This archive is in Loughton Library, North East London. The website has information, news (including events) and historical records. You can search their catalogue and digitised collection here.
World music - News, background a samples of world music, including Rough Guide introductions. Also an extensive online network of musicians and the music trade.
Songlines - Online content from Songlines magazine, covering world music, festivals and events, with good coverage of British and Irish folk.

Encyclopedia of Popular Music - The most comprehensive encyclopedia of popular music, featuring star ratings for individual song collections.
Q magazine - Music news from the popular magazine.
Sold on Song - BBC Radio 2 website, with tips on song writing, guides to genres of pop music, and the history (with streaming audio) of the Top 100 pop songs.
Official Charts archive - Find out what singles and albums were hits and when: search by artist, keyword or date.
Musicals 101 - The 'cyber encyclopedia of musical theatre, TV and film.

Associated Board of the Royal Schools of Music - Lots of information on music examinations, and useful music links.
Music Teachers - Search for music teachers in the UK. This site also includes a range of educational resources (downloadable manuscript paper, exam board notes).

Musiclink - Large selection of links to a range of resources and companies related to the UK music industry. With musician's resources and industry information.
IAML (UK & IRL) - International Association of Music Libraries (UK and Ireland branch) - Represents and promotes the interests of music librarians and libraries, music–related archives and music information providers throughout the United Kingdom and Ireland.

Cecilia - A guide to music collections in archives, libraries and museums in the UK and Ireland.
Encore - A catalogue of orchestral sets of performance music in UK Libraries. Includes the collection held by Westminster Music Library.