These links are our personal favorites:

Search Engines
Ah-ha One Of The Internet's Best Search Engines.
Mobile Audio One Of The Most Complete Car Audio Search Engines.
Yahoo! With over half a million sites divided into more than 25,000 categories, Yahoo! is both browseable and searchable.
MEI Search Directory The Internet Car Audio Directory

Favorite Sites
XSNoise Car Audio The Best Place To Buy Car Audio Equipment
Crutchfield Good source for specific car audio related specs. (Higher priced than XSNoise)
Car Stereo.com Car Audio related forum, FAQ's, and directory!
Car Audio Directory The most popular car audio directory on the Internet.
Parts Express For all of your audio tools, parts, and supplies.