Sites without CAPTCHA can easily be abused, but most CAPTCHA mechanisms are annoying and irritant: The reCAPTCHA software of Google is one of the most proven and user-friendly on the market, and the Yireo reCAPTCHA extension allows you to easily implements this in Magento.
Want to see Google reCAPTCHA in action?
I really like this extension, it saved me a lot of time and I did not have to dig into the files to add code. It is also nice that there are all those configurable options for the shopping cart and other form pages - options that my client can activate or not as she pleases.
For the complete instructions and more features see the online installation guide.