http://www.uncontrol.com/ — a flash applet which provides a good collection of nature-imitating mathematical eye candy. Number 16 is beautiful.
I used to write graphics demos on the C-64, which used a lot of this kind of stuff (although a hell of a lot simpler for obvious reasons). It occurs to me that Flash makes writing demos a lot easier; it provides a decent language (scripting as opposed to 6502 assembly), it gives you a good set of drawing tools (anti-aliasing, alpha blending, and 24-bit colour), the hardware no longer limits what you can do in 2-D graphics, and you can even buy software which takes care of the text effects like zooms, scrolling, bouncing etc. In other words, all the cool tricks are done for you ;)
I wonder what demo writers are doing nowadays, as a result? One side seems to be what these guys have done — actually go for really interesting, good-looking effects, rather than just the “how did they do that” factor. I would imagine the other side of the demo “bleeding edge” is doing a hell of a lot of 3-D stuff. (By hand. In assembler. ;)