Anti spam softwares for webmasters

SpamAssassin is a widely used open source anti spam software on the linux/apache combo servers. It uses a variety of local and network tests to identify spam signatures. This makes it harder for spammers to identify one aspect which they can craft their messages to work around. SpamAssassin will give emails a spam score based on a combination of header and body phrase tests, bayesian filtering, DNS blocklists, some collaborative spam project like Razor2, other variables of your own (blacklisting, whitelisting) and more. You can then decide what tolerance level you want to use against your email traffic.

This one is a bit more on the offensive side. You can sabotage the email harvesting softwares that visit your site, disguised as a user or a search engine spider, and scans all possible email addresses for spam purposes. WPoison generates thousands of random unexisting email addresses and fills their lists with useless data. It's a bit of a trap also for search engines, so make sure you add those bogus pages in your robots.txt exclusion rules.