That depends. Do you have a custom registration page in your template, or are you using the default WordPress registration page? Or the Buddypress registration page maybe?
Have a look at SI Captcha as it supports both. http://wordpress.org/extend/plugins/si-captcha-for-wordpress/
OR if you want a cleaner looking captcha try WP-Recaptcha (API key required- free) http://wordpress.org/extend/plugins/wp-recaptcha/
I've used both and haven't had any spam sign ups yet! ... as further security you could install ban-hammer and enable stopforumspam feature that auto-blocks any e-mails on the blacklist from registering!! (do a google search for wordpress banhammer plugin... i'd link you but it won't let me post more than 2 links)