Websites devoted to bringing musicians and music-lovers together.
Not strictly a music communities, ArtsJournal also encompasses dance, theater, visual arts, and more. ArtsJournal is host to Kyle Gann's blog PostClassic, as well as many other music blogs.
NewMusicBox is an online publication by the American Music Center, devoted, unsurprisingly, to new music. Their definition of "new music" is admirably broad: nearly any musician who considers what they do to be "new" is in. Uptown, downtown, jazz, indie rock, folk music -- they all have their place at NMBx. Unfortunately, their focus on American music -- after all, it is the American Music Center -- sometimes feels short-sighted, despite their similarly broad-minded notion of "American".
Sequenza21 calls itself "the contemporary classical musc community," which is an accurate description. Its contributors include composers, performers, and music enthusiasts, giving a wide range of viewpoints. You can find news articles, concert and CD reviews, discussions, individual blogs, and more at S21.