I recently came across a captcha method which was impressive in the way it worked. Am sure that there are more secure captchas, but what makes this one different is the way its deployed.
The most annoying part with captchas is, it causes unnecessary hassle for the real users and in a way acts as a deterrent for conversion and user friendliness. This captcha solves this issue by two methods.
1. It checks if the form submission is taking place quickly or in less time, i.e. less time than what is normally required to fill in the form manually. This way the normal users are unaffected and the captcha is only shown if submission takes place in less time that the actual set time.
2. Another feature it has is placing an an invisible field in the form, this will be filled in by automated bots which will causes the captcha to be triggered.
I wouldn't say its perfect but it definitely enhances user experience enabling general users not having to use captcha and am sure the actual process can be strengthened more.