Anybody remember that game made in 96kb?
http://www.theprodukkt.com/kkrieger#20I guess that may not technically be a demo since it's interactive, but it's amazing what can be done in that little space. There's even smaller stuff around. I remember one that had 3D planets and music in a mere 4KB.
If you know of any cool demos (or other forms of really small files), post a link. I'm talking about specific demos that you found interesting, not a generic link to a site containing a bunch of them.