http://www.trixbox.org: This is the official TrixBox website.
http://www.asterisk.org: This is the official Asterisk website.
http://forums.digium.com: This is Digium's support forums website.
http://voip-info.org/wiki: The most complete wiki site for all VoIP-related queries.
http://VOIPSpeak.net: Kerry Garrison's site containing articles, news, and tutorials on numerous VoIP topics.
http://www.asteriskdocs.org: This is the home of the Asterisk User Documentation Project.
For further information, you can also refer to the book Building Telephony Systems with Asterisk from Packt Publishing (ISBN 1-904811-15-9) also co-authored by Barrie Dempster.