For me xfishtank would top the list. I still use it sometimes.
Xblast was also cool. It would transform the mouse pointer into a crosshairs and if you clicked it would shoot a hole in the window that you could actually see through. It was really fun to annoy first year students at uni before xauth was implemented.
Xeyes too of course with some --geometry to make it full screen. Did that with a huge video wall once (nearly got me fired but I was just an intern. So worth it)
Xblast was also cool. It would transform the mouse pointer into a crosshairs and if you clicked it would shoot a hole in the window that you could actually see through. It was really fun to annoy first year students at uni before xauth was implemented.
Xeyes too of course with some --geometry to make it full screen. Did that with a huge video wall once (nearly got me fired but I was just an intern. So worth it)