These work by asking a simple question, then the user clicks on the appropriate image. Very easy for humans, very difficult for bots.
My problem with the Confident Captcha system, however, is that it relies on a 3rd party plugin which isn't customisable. I want to be able to set the questions, images and widget display to match the content and theme of my site.
Note: I'm aware that this approach has accessibility issues for blind or partially sited users. For my project, this can be dealt with manually and isn't an issue, but bonus points for options that have a built-in accessible alternative.
Also, bonus points for options that don't depend on a 3rd-party service.
One related module worth mentioning:
- Captcha Questions - totally customisable, but appears to be text only requiring users to do more typing.