I did a survey on this for which to use with clients and came up with http://www.phplist.com/ which I've used a few times and found to be the best of breed OSS solution in this space. YMMV.

It was a while back, http://alicious.com/2008/best-online-mailing-list-management... , and I never really finished the public write up but put it online anyway ... isn't this the space that Google Wave is supposed to be filling now.