An interactive scratchpad
Have you ever been talking over the telephone about something technical (or not) and had to say, "This is too hard to explain over the phone, I really need to draw you a picture." ?
XPHONE is a program that allows you to draw graphics or text on a communal scratchpad shared among multiple users over the internet. The program also allows you to cut out any section of your X display and paste it in the scratchpad.
The idea for this program was originally suggested to me by Harry Orbach at Caltech in 1991. I thought it would be a fun thing to do, so I did it. There are now many other scratchpad programs around of course. The nice thing about this one though is that you don't have to mess around with setting up sockets or other fancy stuff. You just compile it and it works.