Industry:

Audio Engineering Society (AES) www.aes.org
National Association of Music Merchants (NAMM) ww.namm.com
National Systems Contractors Association (NSCA) www.nsca.org
Custom Electronic Design & Installation Association www.cedia.org

Fun:

Blaxxun 3D Community www.cybertown.com
Shockwave www.shockwave.com
USNO Master Clock Time If you really want the right time!