georf

This plugin uses JavaScript to verify that commentators are persons rather than bots.
If a user’s browser doesn’t support or allow JavaScript, so-called CAPTCHAs are used. The commentator has to enter a fruit name given as image before sending his or her comment.

Obstcha

Decent comment verification to avoid spam without user interaction.

Pippin Williamson

Modifies the Empty Spam action in WordPress to process the spam deletion in batches, allowing you to delete thousands or even hundreds of thousands of spam comments at once without killing your server.
Have you found a bug or have a suggestion or improvement you’d like to submit? This plugin is available on <a href="https://github.com/pippinsplugins/batch-comment-spam-deletion" rel="nofollow ugc">Github</a> and pull requests are welcome!

Batch Comment Spam Deletion

Modifies the Empty Spam action in WordPress to process the spam deletion in batches instead of all at once.

Obstcha vs Batch Comment Spam Deletion