I have a few webcams on my network which I can access via URLs like http://198.162.1.30:8080/cam.jpg. Here’s a handy little bookmarklet (pagelet?) which allows you to turn any such webcam URL into live video (here, 1 fps):
data:text/html,<img src="http://198.162.1.30:8080/shot.jpg" onload="var img = this; setTimeout( function(){ img.src = img.src; }, 1000 )">
Paste this data: URL into Safari and then turn it into a Dashboard widget via File > Open in Dashboard. Now you have a video Dashboard widget for your webcam! (Of course you could create a standalone HTML page and load it instead, but data: URLs are cooler and easier!)
WARNING: This could get interpreted as a Denial of Service (DOS) attack if you point this at a URL on a public web server. So it’s best to only do this for cameras on your local network.
Of course, if your camera supports MJPEG aka server-push (multipart/x-mixed-replace), you should just reference that MJPEG URL directly without any such code above.