Brands

Dbx the vintage of the sound
French DBX site
Esoteric
Official site
Luxman
Official site
Revoxsammler
sonystory.com
Studer
Official site
Technics Catalogus

Technology

Compact Cassette Wikipedia
The Story of HIGH COM

Manuals & Brochures

Free Service Manuals
Nostatech's Free Service Manuals goal is to provide free schematics and (service) manuals. So you can repair your old but qualitatively good and often beautiful devices.
HiFi Archiv (German)
A great resource of vintage audio brochures of all brands

Aftermarket products

Akai-Remotes.com
Maker of wireless remote adapters for vintage tape decks
GennLab Alignment Tools
Standard Alignment Cassettes and Standard Alignment Tapes
Nakremotes & FAQ
Nakamichi after market remotes and FAQ pages
RevoxRemotes.com
Maker of wireless remote adapters for vintage tape decks
TascamRemotes.com
Maker of wireless remote adapters for vintage tape decks
TeacRemotes.com
Maker of wireless remote adapters for vintage tape decks

Magazines

HiFi-Advice
Comprehensive audio review site by Christiaan Punter