listhelper is an antispam system developed for GNU mailing lists managed by Mailman. In short, it uses SpamAssassin, invoked via procmail, and some specific mailman configuration settings.
The system is somewhat more fully described on a Savannah wiki page. We've also made an exhaustive list of suggestions and comments about Mailman settings in general based on our experiences.
There are two mailing lists related to listhelper itself. First, email@example.com is for general questions, discussion, bug reports, etc. The main system implementors (Bob Proulx and Karl Berry) are on this list. Second, firstname.lastname@example.org is for volunteers willing to handle the mailman pending queues for a large set of GNU lists.
Savannah project page, source tree (including README).