if you're after a CAPTCHA that validates, and is (almost) as easy to use as reCAPTCHA, please give my SlideCAPTCHA a try. (Wrote it a few days ago, needs some tests in real-life usage.) I based its deployment process on the reCAPTCHA plugin, but you can style it with CSS.
It does require Ruby/GD, however, so if you don't have GD already, I can't promise that GD is easy to install and use!
