No 4. Akismet

This useful plugin checks your comments against the Akismet web service to decide whether they are spam or not. It is a pretty popular and highly rated plugin that is very helpful.

Go here to download.

No 3. Secure WordPress

This plugin gives you the option of securing your WordPress installation by removing error information on the login page. It also adds index.php to plugin directory apart from other security measures.

Go here to download.