Anti-Captcha is a transparent spam solution that does not require any end-user interaction.
It is based on a nonce key, which is dynamically inserted using randomly generated (and obfuscated) javascript.

The aim of this plugin is to prevent automated attacks (by bots) on the following WordPress actions:

• Posting comments
• Registering for a new account
• Requesting a lost password

When a comment is posted without a valid Anti-Captcha token, it shall be instantly marked as spam. This way, you can always manually approve this comment in hindsight if it appeared to be sincere.