SlothCam

Slothcam is the only fully customizable webcam viewer for Dashboard. Just type your favorite webcam image URL into the preferences (or use one of the built-in webcams) and watch your favorite webcam from the Apple Dashboard environment.
Slothcam features include: The ability to add and delete cameras, each with their own refresh rate.

The ability to cycle between cameras when dashboard is activated, at a set interval, or by pressing the arrow keys.
There is also now an iOS version of SlothCam for iPhone, iPad, and iPod Touch! Check out the SlothCam website for an app store link if you are interested!