Project Guitar - This is the best forum on the net for ways to build and modify guitars. I am an Advanced Supporter of these guys and really love the quality of their site.
Wood Descriptions - This page explains the acoustic differences of various woods.
Every Guitar Chord - This page contains EVERY guitar chord....Caution, there are so many, it is hard to find what you are looking for.
Guitar Tab Universe - This is my favorite tab site. Very few ads - tons of excellent tabs.
Ultimate Guitar Tabs - This site is a great all around guitar tab site. Sometimes the ads are a little overbearing though.