The most effective mail classification tool I have ever tried. My accuracy is around 98% (this depends also on your bucket configuration). The only thing I (personally) miss is a phrase classification vs the actual word bayesian classification. The first setup is not so straightforward, but you can rely upon user manual and forum on the official website. It works with POP and IMAP, it is multi platform and it is freeware.
POPFile
pistarino reviewed on 18 Apr 2012
BusyCal
pistarino rated on 03 Dec 2011
[Version 1.6.1]
1Password
pistarino rated on 17 May 2011
[Version 3.5.9]