I have posted about this before (see The Difference Engine), but if you look at HPWREN's display of the Hale Telescope webcam you can get some cool displays when there is activity going on in the dome.
Here is a shot from a few minutes ago:
and the resulting differential image:
The differential image is produced by subtracting the newest image from the previous one. This is the same technique that is used by the Palomar Transient Factory (PTF) as they hunt for supernovae and other other variable sources. See my post What Does a Transient Look Like? for details on how that works.