British Newspaper Archive – has a good search function and contains lots of information about G&S, the operas, performers, etc.
Christopher Browne's Gilbert and Sullivan Online Catalogue – the website for Gilbert and Sullivan music and memorabilia. Many different items are for sale, including books, libretti, sheet music, postcards, posters and recordings.
The Gilbert and Sullivan Archive – was established in September 1993. It includes a variety of G&S related items, including clip art, librettos, plot summaries, pictures of the original G&S stars, song scores, midi and mpeg audio files, and newsletter articles. New items are added regularly.
Gilbert and Sullivan Discography – Marc Shepherd's excellent discography includes details of recordings of G&S, including 78s, vinyl records, CDs and videos.
Gilbert and Sullivan Festivals – the home site for the International G&S Festival
Grim's Dyke Hotel – the website for Grim's Dyke Hotel, former home of W.S. Gilbert.
gsopera.tv – a streaming service which features the very best of The National Gilbert & Sullivan Opera Company and many other leading professional and amateur companies from around the world.
Savoynet – an unmoderated electronic mailing list dedicated to discussion of W.S. Gilbert and Arthur Sullivan, the operas they wrote, British culture and musical theatre of their time, and related matters.
Sir Arthur Sullivan Society – the website for the Sir Arthur Sullivan Society.
The W.S. Gilbert Society – the website for The W.S. Gilbert Society.