WordPress Plugin: WPBayes
I don’t know if I can call this a pure plugin because you have to modify core files to get it to work, but it still seems like a great idea.
From Priyadi comes his WPBayes plugin for comment spam.
As the name implies, this plugin implements spam filtering using Naive Bayesian classifier. This plugin will automatically classify new comments as legitimate or spam based on past decision done by you.