I have found Ween.com to be the best band site I have discovered yet. Weenradio is also excellent.
"If it were beneficial, their father would produce children already circumcised from their mother. Rather, the true circumcision in spirit has become profitable in every respect." -Jesus, from the Gospel Of Thomas
www.bjork.com is FANTASTIC. they have giant page specials on all of her albums, releases and other things, and even page specials on each song in her greatest hits collection! entire pages, with pictures and interviews... the about and about section basically document everything that's been said by or about bjork in any interview she's ever done. the gigography has reviews, set lists and pictures for every show she's ever done. there's a comprehensive bootography too, incase people wonder what bootlegs are out there. combined with frequent updates, free songs, great layout and a gallery of almost all of her music videos which you can watch for free, this is one great great resource.
the goldfrapp site, though, is just so beautiful visually it makes me want to scream.
The "official Elliott Smith Site by fans" is impressive. The "by fans" disclaimer is somewhat misleading because this is the site that Elliott Smith endorsed as his "official site."
You can find news about Elliott Smith's upcoming, posthumous CD "Songs From a Basement" as well as his biography, lyrics to every song he ever wrote, a dicussion board, a guest book, and much more!
I dont really go to official band sites since they are mostly just advertising, but I remember that the White Stripes official site has some mp3 downloads of live performances. One of them was a performance of "Ball and Biscuit" with Bob Dylan. Unfortunately, the quality made the song virtually unlistenable, but anyway, I appreciate the effort.
the only official band sites I was really impressed by was the menomena one (as mentioned above..) and john darnielle's (mountain goats) blog - lastplanetojakarta.com