RF power tubes and transistors, large inventory.
http://www.rfpowerx.com/
Vacuum tube equations, design and technical information.
http://www.aikenamps.com/TechInfo_2.htm
European electron tube charts with characteristic data, basing diagrams, and application info, includes Russian types.
http://www.ginko.de/user/franz.hamberger/roehren/roehren.html
SND Tube Sales - also sell tube testers.
http://www.vacuumtubes.com/manuals.html
Quite a few good links to tube parts sources.
http://www.mods.com/heath-hifi/links.html
A huge site with lots of useful information.
http://www.du.edu/~etuttle/electron/elect27.htm
British site with some great resources!
http://www.valves.uk.com
This site promotes open sharing of information, understanding and historic appreciation of vintage H.H. Scott (vacuum tube) consumer high fidelity audio products produced from 1946 through 1966.
http://hhscott.com