Mine's a 3d flame fractal that I made in Apophysis 3.08: 3d hack. It was the next-best thing to the ESS logo and it just happened to look better in the tiny avatar spaces here than did my ESS logo.
EDIT: This is odd since the fractal image is HUGE and my logo is not much larger than these avatar spaces...oh well.
Mine is a chart showing 6-bit colors spread over 24-bit that I drew by hand on paint. I just like it for some reason. My favorite color is one of them; 5500AA.