M18 Hellcat - Great site, with some great videos - Check out the time lapse restoration of the M18
G503 - Kinda the base for most people in this hobby. Categories for all vehicles. Click "message boards"
Toadmans tank pictures - Just lots of great pictures for all sorts of Military Vehicles. Great reference pictures.
I have bought few of his CDs
Wikipedia list of all the G numbers for vehicles, D & C numbers for Artillery & Guns plus the other reference numbers
Ammo can marking and related information.
Prime Portal - High quality pictures of everything MV related and otherwise
Allied - Axis Magazine / Books + others great reference pictures of vehicles. Much of David Doyle great work comes out of these guys. David Doyle is an avid MV historian and supporter of this hobby. I have easily 30 full size books
to these simple reference books. Buy this stuff it worth it!
Mullins Jeep Parts - All sorts of US MV parts. Also lots of tank parts
M24 Chaffee - G200 Series of Vehicles