Add Hashcash spam protection to Contact Form 7.
This plugin marries the plugins WP Hashcash and Contact Form 7 thus allowing you to protect your contact forms using captcha-less hashcash spam prevention. Requires both WP Hashcash, or WP Hashcash Extended if you prefer, and Contact Form 7 to be installed and activated.
The main advantage of using this plugin is that bots who attempt to send spam using your contact form will be blocked without inconveniencing human users. Users won't have to enter weird image captchas just to reach you.
See the Hashcash for Contact Form 7 homepage for more information.