Ilya Grigorik wrote up a post on [generating a code swarm visualization from git repositories](http://www.igvita.com/2009/01/27/ruby-swarms-visualizing-rails-git/). He wrote [gitter](http://github.com/igrigorik/gitter/tree/master) to feed the git commit info for the Rails repository and feed it to [code swarm](http://github.com/rictic/code_swarm/tree/master):
Ruby on Rails from Ilya Grigorik on Vimeo.
It's worth checking out the HD version. Git was introduced around 4:40 :)