From the “about” sidebar on the homepage:
GroupServer is a mailing list manager: an email sent to one address is then sent on to many people. In this way it is a list server like GNU Mailman. GroupServer also has an integrated web interface, so you can use the web to post, read the archives, and administer your groups. It is a bit like Google Groups. GroupServer gives you an open source system under your control: your groups, on your site, on your hardware.
GroupServer is flexible: a single GroupServer instance powers this site, all the sites at OnlineGroups.Net, and the E-Democracy.org Forums.
GroupServer is scalable: it handles over seven thousand email messages a month, spread across multiple domains.
GroupServer is stable: it has been in production for over ten years.
GroupServer is developed by onlinegroups.net based in Stokes Valley, New Zealand.