We've found a really simple captcha that simply says:
"I am a:" human tiger elephant.
Where the user has to select that they are a human works well. The list of answers changes on every refresh (but the answer is always human). I think it mainly works because no one else has it. It would obviously be very easy to write a script for our captcha. We're happy with our captcha because we know how to change the question easily if bots start getting through. However if we released it, it would be useless because a script to crack it could be written within minutes, but that makes me think,
What about an ACP control panel that allows the admins to make their own required questions added to the profile? Some people could ask simply logic questions like:
Which one is not an elephant: An elephant, A Bear, An elephant
others maths questions:
2 + 2 = ? 6, 7, 4
There is one captcha that uses images and logic:
The only issue with all of these are, How intuitive are they? Does it confuse the user if they have to answer a different question on every site they visit? But is this better then visual confirmation (which is getting harder and harder to read as they combat bots better).