ReCaptcha

You can use the most popular captcha control "reCaptcha" now!

We suggest you need create an account from here
http://www.google.com/recaptcha/whyrecaptcha
(Note: the control will still work if you don't sign up with reCaptcha, but much secure if you do this)

And then create you own domain from
https://www.google.com/recaptcha/admin/create

After that, You will get a public and private key.

And then, you need copy this key to the "Advanced Settings" when you create it

 

You can also choose different theme for it from the "Advanced Settings"
(Default theme is "Red")