If you're the kind of person who loves to get elbow deep in code, there are lots of opportunities to dig into Mailman. Mailman 3 is under active development and we would welcome your input and contributions.
The primary gathering point for all development should be the Mailman Wiki and specifically the Development home space on the wiki. Because wikis are intended to be collaborative, you're free to contribute to this page in true wiki fashion.
Please do not send patches to the mailing lists. They will get lost! Use the project hosting sites described below, and note that they are different for Mailman 3 and 2.1.
Mailman 2.1 continues to be developed on the Mailman project at Launchpad. All patches, bugs, and merge proposals for Mailman 2.1 should be submitted here.
Some of the Mailman developers also occasionally hang out on the #mailman IRC channel on the freenode IRC network. Stop by!