POPfile is an excellent filtering tool that can easily be taught to handle spam. It learns remarkably quickly (even though there is no spam database with it) what your preferences are and is incredibly accurate at predicting how you categorize email. Since it isn't strictly desgined to handle spam it is also incredibly flexible in filtering all sorts of email content as it arrives.
The interface is slightly clunky to some as the settings are accessed via a web page interface but it is incredibly powerful and updated regularly. If you can access webpages from within you email client then the POPfile interface can be a single click away. Note that this is only for setting up the system, once it is running properly you don't need to use that interface as you email client rules are used to filter the tagged messages automatically using the categories assigned by POPfile.
See
http://popfile.sourceforge.net/ for full details.
If you are using Microsoft Outlook (2000, XP or 2003) as your email client there is also a free plugin called Outclass which gives you an easy to use toolbar to access all the popfile functions. If you use this be sure you read the Outclass installation instructions before installing anything as it requires POPfile to be installed but not configured in any way (and not even loaded). See
http://www.vargonsoft.com/Outclass/