Akismet
Akismet — Download from WordPress Plugin Directory
What it does:
- Helps filter out potential spam comments.
- Comments that seem to be spam are held for admin to moderate or delete.
Requires:
- Requires a free API key from Wordpress.com.
- Already have an account with Wordpress.com? Sign in and look for your API key on your Profile page.
- Don’t have an account? Sign up for one. Select the Just a Username option if you don’t want to bother having a blog on wordpress.com.
- More information on Wordpress.com API Keys.