Free Video-Based Captcha: NuCaptcha
NuCaptcha is a free captcha service that uses motion video to authenticate human web interactions.
Compared to image-based captchas, it is a harder-to-recognize solution for bots and can be read easier by humans which is great.
With the help of an API, NuCaptcha can be implemented into any website (sample codes provided) and there is also a WordPress plugin offered.
The captchas can be customized in means of skin, background and message displayed which helps a better visual integration with the websites.