Form Spam

http://u229.no/stuff/Captcha/

"A DEAD SIMPLE SOLUTION TO THE SPAM PROBLEM:
Add a fake form element to your form, make it a text area because spam bots seem to love these.



Then hide the text area using css:
#MySpamTrap {display: none;}

Then check if there is any content submitted with this field. If it is, it's spam."