Musical Heaven is a very complete site with Sheet Music, Memorabilia, DVD's, theatre groups and much more.
Broadway.com is a site to purchase tickets for Broadway shows in New York and in London. It also has a lot of wonderful information under "Broadway Buzz" including headlines, the latest information, photos and more.
Broadway World advertises itself as Broadway's Premier Web Resource, and it just might be that. Reviews, interviews, broadway blogs and much more fill the pages of the site.
The Cyber Encyclopedia of Musical Theatre, TV and Film by John Kenrick can be found at Musicals101.com. This site has a history of musicals, reference, reviews, and lots more. Worth checking out!
At Musicals.net you can find the synopsis of many musicals as well as the songlist and lyrics.
The Gilbert and Sullivan Archive Home Page is, as described on the website, devoted to the works of Gilbert & Sullivan, 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 being added regularly.
Broadway Musicals has an alphabetical listing of many musicals, although not the most recent ones yet, including the basic information on the musical and a songlist which is linked to the lyrics for each song.