After reading recent flame-fest at d-devel, I found crm114 and dspam
seemed interesting.
crm114 is very light and already packaged. I like HOWTO at
http://triplehelix.org/~joshk/CRM114.html
Also check original home page crm114.sourceforge.net
This is supercharged auto-learn grep and very simple design.
Within a day of learning, I get very good filtering. I dropped almost
all procmail recipe filtering junks.
I was quite impressed it works with mixed Japanese/English environment.
This is by far the excellent filter you can teach without programming.
For DSPAM which uses database as backend. Too fancy for me but its web
page is very informative.
www.nuclearelephant.com/projects/dspam/
This has web server support and alternative web support is
provided by php backend, http://www.michaelthompson.org/dspam/