Just wanted to give a mention to Akismet, a spam block plugin for WordPress. Akismet is basically a database built on analysing spam from countless blogs. You can help train the filter, which makes it more effective over time. Those of you using Thunderbird‘s built-in junk mail filter should be familiar with this. From the FAQ:
How does it work?
When a new comment, trackback, or pingback comes to your blog it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down.
Do I have to maintain a blacklist?
Nope! Part of the idea of Akismet is that you’re always protected up-to-the-second from the latest dirty tricks of spammers. There’s no maintenance, no upgrading, no hassle.
I’ve been using it for about 24 hours and it’s successfully blocked all 19 spam comments until now, with no false positives. For those of you using a different platform than WordPress, no worries, Akismet also offers an API, enabling it to be ported to any other system.