I use spamarrest its GREAT ... a lot of people on the otherside find it annoying because they have to enter in the image key. but as long as I only get wanted email in my inbox I am happy with it.
if one happens to be lucky enough to run there own mailserver... it would be good to run a combination of spamassassin, clamav [inline antivirus] and amavisd [all running under postfix].
the combination of the three tools has reduced the amount of spam in my network by a good two thirds or three quarters.