Invisible Captcha
The Invisable Captcha, is an email validation control that requires no user input. The control performs arthimetic on two randomly generated numbers and if correct, the form is submitted.
This is based upon the idea that most spam bots do not currently have the ability to evaluate JavaScript and thus will not be able to submit the form correctly.
If a user has JavaScript disabled then a question is displayed on the screen, allowing the user to type in the correct answer. Below is an image of how the control would look in a JavaScript disabled browser.
Below is an example form, which uses the Invisible Captcha Control.