Piano Websites or Blogs:
Collaborative Piano Blog (great blog with loads of information on the art of collaboration at the piano)

Online Sheet Music Retailers:
Sheet Music Plus 

Vocal and Opera:
Opera Today
National Association of Teachers of Singing (NATS)
VSNATS (Valley of the Sun chapter of NATS)
Phoenix Boys Choir (wonderful musical training for boys aged 6-14)

General Music Interest Websites:
Slippedisc.com  (lots of good articles on classical music)
Arts Journal (general music and arts)
Music Teachers National Association (MTNA)

Universities and Colleges:
Arizona State University Music
Westminster Choir College