Mailman is most likely the most popular mailing list manager in the FLOSS ecosystem. What I like about it is the ease-of-use it provides for the end-user: if u wanna subscribe to topic@host.org mailing list, you simply append -request to the recipient name (topic-request@host.org) and use the subject "subscribe", and simply send. To unsubscribe, you do the same, except the subject becomes "unsubscribe". There's also the web interface for those who won't remember/know these simple instructions of course. Another bonus: it's written in Python!