The internet is a mine of information for audioholics, and some of it is actually useful.
Here are some of the sites I've found particularly good:
Linkwitz Labs - Everything you've always wanted to know about dipole speakers
HT Guide - Especially useful for DIY planar and dipole speakers
Madisound - The best speaker building site on the Net
DIY Audio - Also has useful info for builders
Home Theater Forum - DIY sub central
AVS Forum - The best forum for general HT, especially projectors and HTPC
LSDG - A great resource for information about drivers
Audio Asylum - Know thy enemy
DIY subwoofers - Lots of subwoofer info
AV Guide - Great music reviews from The Absolute Sound, silly equipment reviews
Audiogon - The best used equipment site on the Net.
eBay - The best auction site if you know what you are doing
eCoustics - Good compilation of online reviews
(Sorry about the alignment problem, but I like the colors)