1. Akismet
Checks your comments against the Akismet web service to see if they are spam or not. One of the best in the list.
2. Spam Karma
Spam Karma is a great free replacement for Askimet. If you like to be different from the rest, try this.
3. Contact Form Spam Killer
Spamming is going to happen everywhere and that is where this little plugin will help clear those trying to play punk in your contact form.
WP Contact Form with Spam Protection | Download Direct
4. Defensio
In order to use this effectively, you should deactivate Akismet and/or any other anti-spam plugins before using this.
Defend against spammers Download
5. Simple Trackback
Simple Trackback Validation Plugin for WordPress 2.x performs simple but very effective tests on all incoming trackbacks in order to stop trackback spam.
Simple Trackback Validation Plugin
Recent Comments