This won't filter email before it hits your server, but if what you're really asking is filtering email before it hits your inbox, then we use spamassassin and it works great.

We have a few public email addresses on our site that aren't obfuscated in any way, and I'd say spamassassin misses maybe 1 or 2 spam messages every couple of days without any false positives. Granted, it will take some work to get it properly configured for your particular setup.