This is one of the most in-depth guitar tone sites. You can learn everything from basic tone principles, to the fundamentals of distortion and amp simulation, to highly detailed amplifier analyses, to speaker information, to effects placement concepts, and much much more. This is a staple.
One of guitar websites from the early days of the internet that offers extensive guitar information such as user reviews, classifieds, links, effect explanations, discussion forums, etc. A good resource to use especially when looking for classifieds and user reviews of guitar gear.
Legendary Tones was founded by David Szabados, who has collaborated with Robert Keeley to create the Time Machine Boost. His site is dedicated to helping guitarists find the perfect tone for them, drawing on his year of experience and own personal struggles to find his own tone.
Tone Hunter offers an array of online guitar resources to help with guitar tone, reapair and maintenance. They have gear reviews, articles, links to dealers, guitar events, online media, a list of hot sites, and other resourceful guitar information.
Tone Frenzy uploads sound clips, images and first-hand reviews of electronic guitar equipment. They give insider advice on what gear sounds like, and also sell a few boutique guitar products.
This is one of the best magazines for vintage guitar tone, but they also cover some boutique guitar gear as well. They have in-depth articles covering everything from guitars to pedals to amps to speakers.