This Recaptcha plugin for both Joomla 1.0 and Joomla 1.5 provides an api for easily adding Recaptcha security to a website.

Also comes with the option to add a captcha to the contact page.

View the Installation and Configuration page for information on getting started.

View the Documentation for developer usage.

If you are installing this for use on a Contact Page in Joomla 1.0, please see this guide.