In Tiger, invoke the Dashboard, hit Command-Shift-4, and then hit the Space Bar. Place the camera icon over your chosen Widget and hit return. A .png file of the Widget will be added to the desktop.
I realize that this is the same as the regular behaviour for screenshots, but I thought it worth pointing out that it functions when Dashboard mode is active as well.
I used it to email the weather widget to a friend so I could show off just how sunny it will be in my soon-to-be new home!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050620052055550