Automattic’s wonderful spam fighting plugin for WordPress, Akismet, is available for 24 different systems/platforms according to Matt & Co.
The latest one is a “mod” (they’re called that) for one of the best open source forums out there made by Per Soderlind (of ImageManager fame – a favourite of mine). PunBB is the name of the, er, forum, and now you can fight spam there as well.
It’s nice to see Akismet’s branching out. The fight against spam isn’t just in your inbox and on your blog – it’s everywhere and the more trouble the spammers have to go through the better for everybody else. Go get them!

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I have to say, SpamKarma2 is as handy if not better than Akismet, and they also have a SK2+Akismet plugin that makes use of both filters, 2 is better than 1 ;)
http://unknowngenius.com/blog/wordpress/spam-karma/
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Techz
got to disagree with that, for starters SpamKarma uses Captchas and these are very, very bad for people with disabilities. Overall though Akismet is really the only place to be if you’re using WP and want spam protection.
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Actually, Spam Karma 2 can do the work without using captchas. I’ve found that really spam-infected blogs benifit from using both Akismet and Spam Karma 2, although I personally haven’t had the need to run them both more than during shorter periods of time.
They are both excellent plugins.