Captcha in mobile application and web application
reCAPTCHA accepting one word out of two
implementing captcha in ipad
Need suggestions/ideas for easy-to-use but secure captchas [closed]
Stop spam without captcha
Captcha Alternative, how secure?
Can you run your own reCaptcha service, in your own web app?
Stopping spammers from creating accounts (reCaptcha not doing the trick)
should we use a captcha for Sign Up?
prevent bruteforcing of captcha
Is a captcha enough to enforce multiple failed login attempts? [closed]
Breaking a simple Captcha [closed]
alternative captcha methods
How to Prevent SPAM without CAPTCHAs or a Centrally managed system (e.g. akismet)
How to Verify whether a Robot is Entering Information