I actually find that a simple Honey Pot field works well. Most bots fill in every form field they see, hoping to get around required field validators.
http://haacked.com/archive/2007/09/11/honeypot-captcha.aspx
If you create a text box, hide it in javascript, then verify that the value is blank on the server, this weeds out 99% of robots out there, and doesn’t cause 99% of your users any frustration at all. The remaining 1% that have javascript disabled will still see the text box, but you can add a message like “Leave this field blank” for those such cases (if you care about them at all).
(Also, noting that if you do style=”display:none” on the field, then it’s way too easy for a robot to just see that and discard the field, which is why I prefer the javascript approach).