Mailman, the popular GNU Mailing List Manager. Download/more info at: http://www.list.org/
"Mailman is software to help manage electronic mail discussion lists, much like Majordomo or Smartmail. Mailman gives each mailing list a unique web page and allows users to subscribe, unsubscribe, and change their account options over the web. Even the list manager can administer his or her list entirely via the web. Mailman has most of the features that people want in a mailing list management system, including built-in archiving, mail-to-news gateways, spam filters, bounce detection, digest delivery, and so on."
NB the correct spelling of this software's name is Mailman; the corresponding wiki name is MailMan, used for this page. Use square brackets to link it as Mailman if you like.
See also: MailmanMonitor, ZMailman?, ZwikiAndMailingLists