listhelper is the name for a simple antispam system developed for GNU mailing lists managed by mailman. In short, it uses SpamAssassin, invoked via procmail, and a particular mailman configuration. The system is somewhat more fully described on a savannah page.
There are two mailing lists. First, listhelper-discuss@nongnu.org, for general questions, discussion, bug reports, etc. The main system implementors (Bob Proulx and Karl Berry) are on this list. Second, listhelper-moderate@nongnu.org, for volunteers willing to handle mailman pending queues for a variety of random GNU lists.
Savannah project page, source tree (including README).