Seconding the reCAPTCHA recommendation - I can't think of a way to make it any simpler than this. It even works if you're developing from localhost.

I put off implementing our captcha for a long time, believing that it would take a day of research, implmentation, testing, debugging; with reCAPTCHA and ambethia's Rails plugin (http://github.com/ambethia/recaptcha/tree/master), it took less than half an hour.