Here is a small list of links that you may find very useful.
Harmony Central - Harmony Central is the biggest music site on the net
iBreatheMusic - Instruction, lessons, and more
Musician Forums - Really good forum has just about anything you want on it
Power Tab - Someone from GKN turned me on to Powertab now its the only place I go for tabs
Werner Icking Music Archive - Huge gallery of free classical sheet music
Whole Note - Another big site with lots of info
The Guitarists Network - Great online guitar community
Guitar World - They have a good lesson section and a good message board
Intermusic - This is the messageboard for Guitar Techniques, Guitarist & Total Guitar, three UK based magazines
Guitar BT Forum - Backing tracks for lots and lots of tunes, a lot of these are really good. (You have to sign up for their forum to see the tracks)
6StringHeaven - This is a relativly new message board but they are growing fast and have some really good sources and great people
Vai.com - Good forum, there is a lot of technical info
The Shred Zone - Good site for you shred heads, great lesson section too
The Musical Instrument Makers Forum - This site can help with just about anything you need to know about guitar hardware
The On-Line Guitar Archive - The original Tab archive
MusicBizBuzz - This site has links and information for anyone who plans to make music their business. Information includes record industry contacts, legal info, music jobs and much more.
Jeremy Driscoll - Member of GKN Community and a cool site.