Yes there are.

A Windows Shareware version: http://www.stargraphics.com/ambush.htm

An Android version: http://www.stargraphics.com/amband.htm

There's an applet at boardspace.net: http://boardspace.net/english/about_rithmomachy.html

That was my first thought, too. A cursory search didn't yield anything. I'm tempted to suggest it to my friend as our next project - we're going to do a game project every month this year (or at least: trying to, already behind schedule).

There's a board game: https://www.youtube.com/watch?v=s-H4q6OXSc4

I helped write a parallelized version for the CrayT3E in high school. Our team described the implementation here:

http://www-pgss.mcs.cmu.edu/Publications/Volume18/Rhythmomac...