echhi: that assumes you use one that matches patterns.
dspam and crm114 are statistical, meaning, you train them. my false positive rate on dspam is probably 1:4500. They are server based but there is a popserver proxy, so, you could check mail through their proxy which would pass it through the spam filtering. There may be companies offering that service. Email is again useful for me and, I don't think there is a better way out there than dspam.
http://nuclearelephant.com/projects/dspam/
http://crm114.sourceforge.net/ (I loved the Dr. Strangelove reference here. Funny how that reference keeps coming up eh Cleo?)
The biggest thing, by far, was adding Greylisting. I would have to say that knocked out almost 80% of the spam right off the bat. The downside is that any 'new' person that contacts you will have thier first email delayed 5-30 minutes. Any subsequent email for whatever timeout period you specify will come through immediately. The benefits far outweigh the detriments.
http://projects.puremagic.com/greylisting/