I compiled MathMap for the Gimp today and discovered a neat randomized emboss effect:

p0 = origValXY(x, y);
p1 = origValXY(x - rand(1,4), y - rand(1, 4));
rgbColor(red(p0) - red(p1), green(p0) - green(p1), blue(p0) - blue(p1))

If you replace the rand(1, 4) with a constant factor, this is a traditional emboss--nothing but a high pass filter (one which picks out "high frequencies", or sudden changes in the image). The randomization causes the embossing to be somewhat more rounded and "weathered" around the edges.

(The original version of this post can be found on the Wayback Machine.)