I stumbled onto these guys a few years back when I was going nuts about base 18 (which you can do on a standard 5x2 abacus!). I think they have their own symbols for digits > 9 instead of using 'a' and 'b' in the hexadecimal fashion.

Non-standard radixes are pretty fun, and it makes you think about nebulous questions like "what does ten mean", and if you happen to carry that train of thought way past the station, "what does it mean to be a number?"