Easy Mailing Lists:
It has programmer tests (of course), customer tests (a new feature I'm trying), and a README file that describes everything you need: your own mail server, a user account, a couple of other Perl modules, a .forward file, and resolveable extended e-mail addresses.
We've been using it successfully for a while here, but in packaging it up, I may have messed things up. The tests all pass, but I've been writing them long enough to know that's never the whole story. There are also plenty of features to add, but it's been interesting to write, so why not share with the rest of the world?