The original recaptcha library can be downloaded from here

Comments or suggestions: brianpow [AT] gmail [DOT] com

First, register a recaptcha account to have the private key and public key.

Then, edit the config/recaptcha.php and add your keys.

To check the answer in Controller:

$recaptcha=new Recaptcha(); 
if($recaptcha->check($_POST))
    $message='Answer Correct!";
else
    $message='Are you robot?";

You may also get the original error message by

$error=$recaptcha->error();

In case you want to use more than one key in your project, you may add your new key as parameter when creating a new object

$another_recaptcha=new Recaptcha($my_other_keys_array); 

To show the input field in View:

$body=View::factory('YOUR_VIEW_FILE')->set('recaptcha', $recaptcha->html());