A static image like this doesn't really do it justice. The colors sort of slide
along its length, giving the illusion of motion. I just did rectangles; then someone else came along and added other shapes, like the hexagon you see here.

I also did most of the "mandelbrot" mode in
xlockmore. This is a subregion of
a "power 3" mandelbrot set. Actually, I have no idea if this is officially
called a "power 3" mandelbrot set; I just took the mandelbrot set equations
and changed the exponent from 2 to 3. The mode does a variety of exponents,
with a random-but-interesting subregion selection. It also slides the
colors along; the code for the color cycling is derived from the color cycling
in the above "tube" mode.

And I did the "ellipse" code in xlockmore. Strangely, this is the
only code of mine that got ported into
xscreensaver, even though I
personally like tube and mandelbrot better. This is very basic
trigonometry. Here's a picture: