What Captcha do you use for your Drupal 6 site?

1. place phpcaptcha in sites/all/modules/phpcaptcha
2. modified securimage_show.php
   
   <?php
// START MODIFICATION
// added to allow Drupal to initialize it's session
$currdir=getcwd();
chdir($_SERVER['DOCUMENT_ROOT']);
require_once("./includes/bootstrap.inc");
drupal_bootstrap(DRUPAL_BOOTSTRAP_SESSION);
chdir($currdir);
// END MODIFICATION
include 'securimage.php';

$img = new securimage();

$img->show(); // alternate use:  $img->show('/path/to/background.jpg');
?>
3. You probably want to make the same modification to securimage_play.php
4. Our form was some simple php code as the content of a node, and looks like this (simplified to protect the innocent ;-)):
   
   <?php
include_once $_SERVER['DOCUMENT_ROOT'] . "/sites/all/modules/phpcaptcha/securimage.php";

if(isset($_POST['send'])) {
    $securimage = new Securimage();
    if(! $securimage->check($_POST['captcha_code'])) {
        print "<div class=\"messages error\">Captcha error</div>";
    } else {
        print "<div class=\"messages\">Captcha correct</div>";
    }
}
?>
       <form name="test" method="post" id="test">
        <table>
            <tbody>
                <tr>
                    <td><img id="captcha" src="/sites/all/modules/phpcaptcha/securimage_show.php" alt="CAPTCHA Image" /></td>
                    <td><input type="text" name="captcha_code" size="10" maxlength="6" /></td>
                </tr>
                <tr>
                    <td colspan="2"><input type="submit" name="send" id="send" value="Check" /></td>
                </tr>
            </tbody>
        </table>
    </form>

I hope this helps, good luck.