About: Guitar: About.com's site for the guitar - this is a very informative site!
11 Guitar Lessons Online: From About.com's guitar page, these lessons are a pretty good primer to 'teaching yourself.' They can provide a good base before
signing up for lessons.
Chordie: Free guitar tabs and chords for songs. Huge searchable database
Cipher: From their website, "The Cipher System is a new method for studying and teaching the elements of Western music theory and the guitar fretboard (among other stringed instruments)."
Keyboard Trainer: An fun and interactive way to learn the notes on a keyboard
Music Dictionary: The Dolmetsch Online Music Dictionary
Music Sheaf: A site where can print out blank manuscript paper
Music Theory For Songwriters: A nice little resource that explains creating chord changes in a no-frills way.
OpenMikes.org: A site I'm just checking out that lists all sorts of open mikes to play at.
Ricci Adam's musictheory.net: A great online resource for all things theory related
wikifonia: "Wikifonia is the place for musicians to publish and collaborate on lead sheets."