At Google we use a code review tool which was open sourced a couple of
years ago: Rietveld
(http://code.google.com/appengine/articles/rietveld.html).
The best way of thinking about it is "bugzilla for patches". The
system creates an entry for every patch submitted, provides a web tool
for manipulating the patch (comments, different views of the diff,
highlighting, etc) and it also has an email gateway.
We have discussed patch tracking mechanisms in the past, and none so
far has taken hold. The reason why I like Rietveld is that it doesn't
really matter whether we all switch to using it at once:
1- Rietveld always send the patch sent to it to gcc-patches@ (provided
the submitter added gcc-patches to the CC list).
2- The whole trail of discussion on the patch also get sent to
gcc-patches and everyone else is CC'd in it.
3- Reviewers do not need to use the web tool to reply to the patch.
One can simply respond to the e-mail, and it will get added to the
patch discussion trail.