Dirk's Guitar Page - A treasure trove of great (and free!) classical guitar sheet music both in tablature and standard notation. Some of the arrangements here are not the best, but nevertheless this is a great archive of hundreds of the most popular pieces in the classical guitar repertoire.
Classical Guitar Blog - A continuously updated blog of inspiring classical guitar videos, practice tips, musical advice and more!
ClassicalGuitarMIDI.com - Listen to how those classical pieces you're working on are supposed to sound. This is a large archive of MIDI files of works from the classical guitar canon.
FREE Online Guitar Tuner - A videoon the same page will show you how to string your guitar.
FREE Online Metronome - with selectable tempos from 40 bpm (largo) to 208 bpm (prestissimo).
Guitar Delicacies - MP3s and tablatures (as PDF and Zip files) of famous traditional Flamenco falsetas, Classical guitar pieces, Jazz standards, and Rock songs.
A History of Music Notation – Read this article to learn about how music notation has evolved from ancient times to the present.
Music Theory Basics– An article on the fundamentals of theory and harmony.
Musictheory.net - A very educational interactive music theory program. Warning: you must know your music fundamentals to understand this site!
Music Theory Note Trainer - Click this link to test your knowledge of notes on the guitar fretboard. See how fast you can choose the correct note!
History of Jazz Guitar– A brief overview of the Jazz style with links to Jazz guitar sites and artist pages.
History of the Classical guitar – From the medieval lute to the modern Spanish guitar, the story of all modern guitar genres begins here.
The Electric Guitar – Learn what an electric guitar is made of, how it works, and its history.
Sheet Music Plus– Order instructional books and sheet music online from this Internet music store.
Powertabs.net – A continually updated archive of guitar scores for all styles of music.