I first saw this widget earlier in the week at Macworld when someone sitting next to me pulled it up on his dashboard. Radar in Motion
pulls a radar image of a user-specified location (either zip code or a more general region) from The Weather Channel and displays the animation. Click it while its running to zoom in for a larger image. Click again and it zooms back. Preferences include refresh rate, animation speed and a slew of maps to choose from. Check it out, it's fun.