We have several mailing lists devoted to Mailman, which also provide a nice demonstration of Mailman!
Mailman Users is the mailing list to subscribe to if you are using Mailman at your site, and may have problems or questions relating to installation, use, etc. We'll try to keep the deep technical discussions off this list. (*archives*)
Mailman Announce is a read-only list that you can subscribe to if you are only interested in release notices and other important news. Only the core Mailman developers can post messages to this list. (*archives*)
More information about developer resources is available here.
Mailman Internationalization is the list for discussing the multi-lingual support in Mailman 2.1. Everyone who is working on translations of Mailman should subscribe to this mailing list. (*archives*)
Mailman Checkins is an adjunct list to the publicly accessible, read-only, official Mailman 2.1 Bazaar branch. This list is for the hardcore developers, or anybody else submitting patches, since we really prefer such patches to be generated against the latest snapshot. This is a read-only list; only the core Mailman developers can post messages to this list. There is no archive.