As far as I read, ASSP is a great competitor to SpamAssasin. I will give it a chance on my test server and I will generate a report about it.
ASSP with ClamAV (make sure you use the Sane Security sigs!) will cut your spam to practically none.
ASSP – http://assp.sourceforge.net/
MAIA seems to be an SpamAssasin-based solution.
Maia Mailguard is a web-based interface and management system based on the popular amavisd-new e-mail scanner and SpamAssassin. Written in Perl and PHP, Maia Mailguard gives end-users control over how their mail is processed by virus scanners and spam filters, while giving mail administrators the power to configure site-wide defaults and limits.
Very nice presentation about open-source anti-spam solutions: