Due to the insights of our commentors, I've changed my mind about circular treemaps. They have a distinct disadvantage compared to rectangular treemaps in that the parts (sub-circles) do not add up to the whole (the surrounding circle). But circular, colorful data representations still appeal to me.

Ciros circular graphs (shown above) are quite appealing. They represent a method for visualizing genome relations, but could be applied to any complex relational dataset. It is also an free GPL project, so you can try it out for yourself. For inspiration, checkout these lovely screenshots.