GraphViz.pm is a Perl wrapper for the excellent GraphViz library. Installation is easy, download Graphviz for OS X, which is adds a slick GUI on top of the command line tools, drag it to Applications, and add /Applications/Graphviz.app/Contents/MacOS to your $PATH (probably want to do this in your .profile so it persists). Now perl -MCPAN -e 'install GraphViz', and your done.

What I want (and I’m surprised doesn’t exist) is a Graphiz to XHTML/CSS renderer, for small to medium sized graphs seems obvious.