Finally, a captcha you can't read...wait...that's not what I meant... This is the captcha you don't even need to see.
Originally proposed by Adam Beck in 1997, HashCash requires a form to include the solution to a complex calculation. The calculation is so complex (it takes hundreds or even thousands of attempts to solve it) that any human or bot attempting to abuse your forms will spend so much processor time solving the calculation that it wouldn't be profitable to continue attacking your forms! The server receives the result and can easily and quickly test it in 1 calculation - either it's right and your user continues or it's wrong and the form submission fails.
The best part is, HashCash is invisible (you can't read it - or even see it) and it happens in the background without user interaction! Your users arrive at the form and the HashCash calculation is automatically executed. Any bot attempting to submit your form without completing the calculation is rejected, and the calculation changes every time the form is submitted.
Configuration is simple. Open the plugin and choose the difficulty level. The predefined minimum (1) and maximum (4) levels prevent calculations that are too simple or too difficult to complete in an acceptable amount of time.