Warmoth: comprehensive supplier of guitar and bass parts
Stewart Macdonald: hardware, electronics, and other; some tutorials
Guitar Reranch: refinishing supplies and tutorials
Allparts: comprehensive supplier of guitar and bass parts
eBay: if you don't know ebay, I don't know what to say
craigslist: usually some good deals to be found in the instruments forum
Seymour Duncan: pickup descriptions and sound samples; they also have a forum
Robert Keeley Electronics: mods on effects pedals
Zzounds: good prices on new gear and some hardware
Harmony Central: comprehensive musician's site; reviews, classified ads, etc.
guitarra thread: scroll through thousands of posts, or just ask a question
The Gear Page: gear forum; I haven't used it much, but I've heard it's good
Kisekae: very cool applet that let's you build custom guitar images (click "build your dream guitar")