It's no secret that the use of difficult CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart) in Web forms can confound ordinary people, drop conversion rates, and increase errors in addition to (hopefully!) keeping the bad guys out of your site. As a result, I'm always interested in alternatives to standard CAPTCHA techniques.
Once such approach brought to my attention recently can be found on They Make Apps. Instead of the distorted text strings that typify most modern CAPTCHAs (above), the sign up form on They Make Apps uses a slider that asks people to: "show us your human side; slide the cursor to the end of the line to create your account." Moving the slider to the right completely submits the form and triggers error validation just like a standard Submit button would.
I don't have any data that suggests this implementation impacts conversion rates (up or down). It doesn't seem to cover accessibility issues since it requires a mouse or drag gesture to work (no keyboard support). And at large scale, it won't keep spammers out effectively. CAPTCHA busting farms already employ human labor to crack distorted text CAPTCHAs at incredibly cheap rates (moving a slider would be even faster & easier) and it's likely trivial to develop a script that will adjust the slider automatically to submit the form. But it is still nice to see people exploring different approaches.