WP-reCAPTCHA by BlaenkDenum integrates reCAPTCHA anti-spam methods with WordPress including comment, registration, and email spam protection. WPMU Compatible and reported to work OK with BuddyPress.
reCAPTCHA is an anti-spam method originating from Carnegie Mellon University. reCAPTCHA is a free CAPTCHA service that helps to digitize books, newspapers and old time radio shows. http://www.google.com/recaptcha/learnmore
Download and info: http://wordpress.org/extend/plugins/wp-recaptcha/