I have learned more about TONE from the following two Websites in the past few years than I did in the previous 30+ years of playing.
The Les Paul Forum. You can view most content using a free membership, but a small subscription is required to view everything.
The Plexi Palace Forum. Free membership. Please notice this is the link to the Plexi Palace Forum. Plexi Palace is a highly-respected vintage amp shop that sells and repairs vintage amps. They also have their own vintage-inspired line of amps.
There are MANY more great websites out there; Here are some of my favorites.
All Things Fender, including guitars, amps, & clones(!).
A very friendly forum dedicated to all things Les Paul.
Remove the "blanket" from your Les Paul's tone! This website has quality replacement electronics and much more for your guitars.
First Class parts for your Fender guitars. "Like they used to make 'em."
Randall Aiken is a Very Knowledgeable & Cool amp guru and a First Class Gentleman.
Cables DO make a difference. The amount of difference depends on the specific cable and the type of music/tone you are playing.
All things Marshall, including the newer stuff.
Information Source for Great Guitar Tone. Includes several reviews.
Tons of Information about Vintage Guitars & how to identify models, dates, etc. Great source for collectors.
An online guitar magazine and general guitar news site with thousands of guitar-related articles.
A guitar blog updated daily with industry news press releases, new gear announcements, and occasional interviews.