>Anyone has used those auto generator engines to generates a number or a string into an image, which we can then use on Web form to ask the user what he sees, so to add additional security to the Web site? I would like any recommendation on that and also as to know if a free version is available somewhere that can do this job?

The technique is called Captcha. I remember I saw a nice article in ASP.NET magazine a while back. Try googling on 'free captcha utility'