ezmlm ("Easy Mailing List Manager", not to be confused with
MLM) is a
mailing list manager program that has quickly become a serious competitor to
Majordomo... It's based on
qmail MTA, and was written by
Dan J. Bernstein.
It has all features you can expect from a mailing list handler: subscription handling, archival, bounce handling and such.
Personally, the only thing I've found annoying about it is that it looks at the real address by default when subscribing, not at the From: address (I guess this is to make listbombing harder). This is a pain in the neck sometimes.
Home page: http://www.ezmlm.org/