Leitz and Leica Equipment:

CameraQuest's Camera Profiles - Lots of information and opinions about Leica (and other) camera stuff.
Farines-photo.com - A good introduction to Leica screw-cameras.
Leica Camera AG - Manufacturer's Home Page
Leica Rangefinder Visual Library - Superb Leica-M and Screw-mount Photo 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.
Pacific Rim Leica Pages - Good resource for early camera information and images.
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 of Leica screw-mount cameras.

Pre-owned Leica Camera, Lenses and Historical Items