Demo groups use various equipment to create their demos. Of course, computers are nearly always used and there's been pretty many different platforms utilized for demo programming. The usual ones are of course C64, Amiga and all kinds of PC combinations.
It wasn't too many years ago that Atari ST was a big rival of Amiga and the sound-lacking Atari had some kind of a demo scene. But if you thought that VIC-20 is not among the computers used for demos, you are wrong. Actually there has been some people more or less seriously trying to code demos on it too - even some people in Beyond Force. You can read more about our equipment in the page: Presentation.
There are some different machinery used for creating spectacular effects - namely videos. Some people use videos and video- cameras for collecting stuff for their computer demos. Some people even calculate animations with a computer and save the frames to a video tape so that the room available won't become a problem nor does the speed of the routines limit the effects.