Akismet provides an unobtrusive mechanism for filtering spam posts from user submitted content. It works. Well. You never have to pester your users with the psychedelic Turing test that is Captcha again.

I installed the Zend_Service_Akismet implementation here and MicroAkismet on a few other blogs over the last week. Even sites with serious comment spam problems have seen the spam drop off to zero. I haven't seen any false positives yet.

Get it. Use it. Love it.