This is the official page of the plugin- http://code.google.com/p/joomla-recaptcha/
Or download from here – plg_recaptcha.8.tar
Another article about captcha in the default Joomla 1.5 contact page here – http://grekai.wordpress.com/2010/02/13/install-captcha-into-default-joomla-1-5-contact-form/