Ways to prevent bots from filling out forms

What are your suggestions for preventing bots from submitting a form while maintaining a good user experience?

Comments

[Jesse Phillips]

CaptCha - really anything you build to distinguish humans and computer is a CaptCha. But it is also the search term to get you started.

[Phil Ecker]

I've done captcha before, but feels to me like that takes away from user experience. Typically they are hard to read. Even things like Google's reCAPTCHA are flawed with it being hard to identify what is in the photos they provide.

[Cristina Ruth]

There are hidden CAPTCHAs now. I forgot the term but Google should show some results.

[Jesse Phillips]

D form's captcha system asks the user to provide a small programs output.

[Autumn]

Sounds like a problem exactly for captcha

[Phil Ecker]

Thanks. I've used captcha, however I'll argue that user experience isn't great with that approach.