Guitar: Wikipedia article describes the guitar family and its history; also links to related articles and external resources.
GuitarNation.com: A virtual gallery featuring a range of custom and hand-made guitars - nylon/steel string, acoustic/electric, archtop, bass, and historic recreations.
GuitarSite.com: A resource for guitar enthusiasts. Includes newsletter, guitar history, interviews, tablature, directory, MP3s and chat.
GuitarWiki: A resource for guitar players, to which everybody can contribute their knowledge. Includes history, information about the structure of the instrument, and tablature.
Harmony Database: For enthusiasts of Harmony guitars, a database of specifications and photos; includes acoustic and electric models, listed by model number, brand name and family.
How Stuff Works: Guitar: Graphic seven-part tutorial on how acoustic and electric guitars work; includes sounds and a page of related links.
Kokomo Music: Vintage guitar collection on display, as well as CD recordings of vintage guitars for sale.
MoMI: The virtual Museum of Musical Instruments focusses on old guitars showing collections and exhibitions with different themes.
The Guitar Pre-1650: The guitar and its predecessors: a small history.
Vintage Guitars and Basses: Information and photos of instruments by major companies like Fender, Gibson and Rickenbacker, as well as many smaller brands.
Vintage Guitars Info: A collector provides an illustrated guide to vintage guitars (acoustic, electric, resonator and steel). Includes related articles.
VintAxe.com: Collection of some lesser known vintage guitars produced from the 1950's to the 1990's with catalog scans, pictures and a message board.