GroupServer [1] is very nice, a hybrid between an online forum interface and a fully-functional mailing list. The installation and configuration process is fairly involved, though, but turnkey hosting is available through OnlineGroups.net [2], from $2/month [3].

[1] http://groupserver.org

[2] https://onlinegroups.net/, https://onlinegroups.net/l/groupserver

[3] https://onlinegroups.net/pricing/