NEW EDIT:
I just implemented something like it: http://jsfiddle.net/ravan/FK4st/2/
EDIT:
A better alternative is to display like 3 squares, one color each, and ask the user to click on a specific color. You can also use multiple divs overlayed with low opacity in order to get color meshs. No difference for user, but a headache for spammers.
Old Answer:
You can ask something to user, like:
What day is today?
What day of the week comes after tomorow?
How much is 2+2?
Notice that it is not as secure as captcha, but should filter some spammers.
