Subjectively

diff2html is a free and open source utility that uses the output of diff(1) to graphically compare two files in html.

A python version of the same thing exists here.  This version of diff2html was created as an exercise in  writing bash string handling and array processing routines.  The python version is actually faster, but I did not want to install python from macports on my iMac just to run this single script.  All things considered, I’m reasonably happy with it, and the performance is not too shabby (it *is* a shell script).  It has taken about 3 days worth of my time to create and improve.  I’m not really sure where it is going from here, since it does a fairly discreet thing and it does it decently.

The permanent development site is located at SourceForge:

http://www.sourceforge.net/projects/diff2html