One of the most popular and very effective methods to combat spam is to use some form of captcha protection. But, captchas can lead to poor user experience. reCAPTCHA by Google is a quite different captcha trying to minimize user interaction.
There are many ways to fight spam. And, spam delivery method will largely influence the way to fight it. Honeypot on the login form is very effective if your website is free for anyone to register.
Before we get to methods to fight comment spam, it is very important to understand how spammers and spambots work, what methods they use, what types of comments they are posting.
Any WordPress website that allows comments or implements forum (bbPress or something else) attracts comment spam. If you ever wondered how comment spam finds your website, this is a good place to start.
GD Security Toolbox Pro is a new Dev4Press plugin dedicated to hardening WordPress security. Plugin monitors security related events and includes wide range of tools to trap spammers and stop malicious attempts.
GD Press Tools Pro 5.0 is coming along nicely, and at this point, all of the plugin features are outlined and already completed or in development. This post lists all the planned features along with plans for GD Security Toolbox Pro plugin.