leonardo_m just posted a fine collection of links to algorithmic art. His link to nodebox attracted my attention first as it is based on Python on the Mac, but the art he shows in his post from chriscoyne is more visually appealing to me: of the fractal L-system type, but with very good control of color, line, and tonality. I also found the image below (from nodebox, created by Tom De Smedt) especially intriguing; it's a visualization of the pattern of motion of a flock of simulated birds, but I'm not sure whether it was intended more as a visual aid for understanding that motion or as a piece of abstract art.
actually, it is really amazing to see an abstract design like these! it has a deeper meaning!