Here we go:

1. Akismet – a very popular plugin which comes by default with wordpress. To use it, you need to signup for a wordpress acount and get an API key for your website.
2. Defensio – A wordpress plugin that helps you filter your comments and also provides rss feed for both accepted and ‘spammed’ comments.
3. Simple CAPTCHA – Adds turing text to your comment area to eliminate automated comments
4. Challenge – Adds a challenge to your comments area, the regular stuff being additions and multiplications
5. Referrer Bouncer – You don’t need to do anything to use it. Just activate it.
6. Email Immunizer – Simply converts all your emails on your website to numeric references to prevent email colecting
7. WPBayes – Implements the spam filtering with the Naive Bayesian technique, which means it marks the comments as spam or not based on your previous decisions. To be honest, I didn’t use this one
8. Spam Karma 2 – This one assigns a karma score based on multiple factors like the age of the post, the email, if there’s a link inside, etc, etc.
9. WP Spam Hitman – It fights agains wordpress spam using a series of patterns. That’s all I know about it, please test it and let me know if it’s a good one.
10. Did you pass Math – Does the same thing as the one at point #4.

Well.. I think 10 tools are enough to keep you away from spam comments. Test them and use the one(s) you like the most.

- Peter