Substitution guide, type, pin-outs and heater voltages for many tubes.
http://www.xs4all.nl/~tgale/valves.html
Some excellent theory, vacuum tube circuits, manuals, intersting stories and information.
http://www.mines.uidaho.edu/~glowbugs/
Interesting tubes site from the United Kingdom. Many pictures, data and some for sale/wanted.
http://www.webiosis.co.uk/valves/index.shtml
Information about electron tubes, and valves, from many parts of the world.
http://www.tubedata.com/
Tube factories and their logos.
http://www.ominous-valve.com/russtube.html
Tubes and parts for Hi-Fi, Radio and Guitar. Tube data and amp schematics online, links to tube amp information on the web
http://www.triodeel.com/
More tube tester source data.
http://www.acadiacom.net/nlee/tv-7_index.html
Tube audio projects and datasheets
http://www.vt52.com/