Chicago 51 - A forum for discussing all things Valco.
ValcoAmp.com - Run by Terry Dobbs, expert on Valco amps.
The Gretsch Pages' Amplifier Page - Info on Gretsch amps made by Valco.
Notecannons - Info on National guitars and other gear, including a lot of pre-Valco instruments; the best background music on the internet.
The Magnatone-Valco Forum - A Yahoo forum for Magnatone and Valco amplifiers.
Pre War Amplifiers - includes pics, info, schematics and a few sound clips for a variety of really old amps, including a few National-Dobro gems.
The Gear Page - An excellent forum for musicians.
Gbase - Searchable online database of instrument dealers' inventories, including new and vintage gear.
Vintaxe.com - A database of vintage instrument catalogs, including many Valco guitars and amps.
Modern successors to Valco and its brands:
I'm not endorsing any of these companies or their products, but each of them is somehow carrying on a part of the Valco tradition.
GuitarKitsUSA - Makes new res-o-glas bodies and hardware along the lines of '60s Valco guitars.
Eastwood Guitars - Makes reissues of a wide variety of quirky vintage guitars, including a number of Valco products.
National Resophonic - Not really connected with the old National/Valco company, this company is still one of the premier manufacturers of resonator guitars.
Supro Guitars and Amplifiers - Made by Zinky Electronics, these are adaptations of old Valco designs with a few modern upgrades.