With Akismet from Automattic or more specifically Matt Mullenweg. I just purchased a Pro-blogger license for an entire year and installed the WordPress plugin on all of my blogs. I removed my other spam prevention plugins, turned off the Comment author must have a previously approved comment option in WP, and even deleted my comment spam word blacklist. That’s how confident I am in Akismet as a spam prevention service. We’ll see how it works out. I doubt I’ll regret it.
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.
If you’re running a WordPress powered blog I suggest you give it a try and support those who are behind it. They’ve done a lot for the blogosphere over the last 2 years.