>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?

reCaptcha offers a WS based implementation -
cost free, helps digitizing books and kept up to date by google paid devs.

if you can follow an article in german:

http://www.heise.de/ix/artikel/Turing-fuer-alle-1033340.html

under (at the bottom)

Links zu diesem Artikel

(links to this article) you can find client side implementations in java,c# and a few other languages

HTH

thomas