I tried many including the commercial ones and I've found this one to be the best for me:
Spampal
It works so well because it uses multiple technologies...
both realtime block list and a Regex filter. The spammers may be able to get by one or the other but usually not both.
If you don't believe me , why not check out the USER reviews at
CNET/Download.com
Oh and its freeware.
