We have been using Google Apps' Message Filtering service for a few weeks now, and all I can say is that it seems to work great.  Although not perfect (and that would be the breakthrough of the Internet age if someone created a perfect spam filter), it works well enough for our needs, as well as some of our customers.

Services start at $3 per year, per email address.  For business users mostly, or those with their own domains/mail servers.

Check out Google Apps Message Security here .