Leitz and Leica Gear:

CameraQuest's Camera Profiles - Information and opinions about Leica (and other) camera equipment.
Farines-photo.com - A good introduction to Leica screw-mount cameras.
Leica Camera AG - Manufacturer's Home Page
Leica Rangefinder Visual Library - Superb Leica-M and Screw-mount camera and lens images.
Leica.overgaard.dk - An excellent Leica resource - be sure to see the camera and lens compendiums.
LeicaWiki (German) - Similar to English language LeicaWiki with many differences.
Marco Cavina - Wonderful lens test site, even better if you know some Italian.
Pacific Rim Leica Pages - Good resource for early camera information and images.
Puts M-Lens List - Useful list of hoods, caps and serial numbers for Leica M-lenses.
Puts R-Lens List - Useful list of hoods, caps and serial numbers for Leica R-lenses.
Summilux (French) - Wonderful, inclusive Leica site, even better if you know some French.
The Leica Pages (German) - Beautiful site for early Leica literature, even better if you know some German.
Andrew Yue's Leica LTM Cameras - Good description and history of Leica screw-mount cameras.

Pre-owned Leica Camera, Lenses and Historical Items