Guitar.com
A bunch of lessons online, plus great other features, like interviews, a chord generator, photo gallery, hot news, and more.

Harmony Central Lesson Archive
Collection of lessons spanning many different topics. Many of the lessons have been submitted by users, so some of the information is less than top quality.

Ultimate Online Guitar Tutor
Well organized, and well-documented lessons aimed at the beginner guitarist. Many lessons include audio clips for clarification.

Artificial Harmonics
Written by Lenny Breau before his death, a great introduction to artificial harmonics, and a glimpse into Breau's playing style.

Bebop Dominant Scale
From Jazz Guitar Online, a short primer on one of the scales used extensively in jazz. Site also includes several pages ( 1, 2, 3) of "licks" based on the scale.

Exotic Scales
A text only explanation of some unusual and rare scales.

Harmonics
From Guitar Lesson World, instruction on how to go about playing artificial harmonics on the guitar.

Jazzy Sounds for Rockers
In an article written for Guitar Player Magazine by Fareed Haque, the guitarist explains several steps that rockers can take to infuse new sounds into their solos.

MAGAZINES

Acoustic Guitar
A few interesting articles on the on-line edition of AG, but you can learn a lot more if you sign up for a subscription to their printed magazine through the website.

Guitar Magazine Online
Robust website that updates with each new printed issue of Guitar Magazine. The online version offers many interviews, workshops, sound advice, and more.

Guitar Player Online
Online version of Guitar Player Magazine. Website offers lessons, guitar giveaway contests, interviews, classifieds, and more.

Guitar World Online
Online version of the popular magazine. Subscriptions to the print edition are available through the website.

Jazz Guitar Online
Strictly an e-zine, this site, maintained by Bob Patterson, provides video reviews, lessons, Q&A, spotlights on jazz guitarists, amongst other things.

Just Jazz Guitar Online
Not to confused with the above, this website offers online lessons, a photo gallery, and various other snippets from their print edition. You can subscribe to their magazine via the website.

PHOTOS

Classical Guitar Photos
Site with archived photos of classical guitars, including separate photos of headstock, label, and rosette for each. Every guitar is available for purchase.

Pictures and Gibson
A fairly small collection of photos, including pictures of ESP, Gibson, Fender, and BC rich guitars.