I've written a simple web GUI for the excellent Ecartis mailing list software. The goal is to allow (non-technical) people to easily manage their own mailing lists. Right now, it allows you to subscribe/unsubscribe people to/from a list, and also list who is a member of a list. It is written using PHP 4.
Ecartis does have a perfectly reasonable CGI GUI, but I feel that it has too many options. The intent of this is to allow the sysadmin to control the configuration options (in accordance with site policies, etc.) while not being bothered about "people" list maintenance.