Once I found a nice captcha replacement (can't remember where it was, tho). It worked like this: at the end of the form, there were 6 to 9 playing cards and the text "Click on the seven of spades". This fitted the theme of website which had something to do with poker or magic tricks (can't really remember what it was) but this can be done with a lot of other stuff that is well known by people and not by robots (e.g. 6 photos of animals and the question "Click on the dog", etc.)