There is a free Google reCaptcha which can be used with WordPress contacts
https://www.google.com/recaptcha/intro/index.html
It creates/registers a site key and secret key for use in the contact integration and the html tags for the header and the contact page. Time will tell how well it works on this site.
Follow up. Integrates with contact form 7, remember to put [recaptcha] in the form.
