Replacing the "captcha"?

To give a link to an IQ test, there are a lot of questions, if you answer in less than 20 minutes to send nafig. If you score less than 90 points — send nafig (either a bot or an idiot), if you type more than 130 — also send nafig (or a bot with answers to IQ test, or a genius, which your site still would be boring).

Unfortunately, the bot can wait 5 seconds and 10. You also should conduct the Turing test, which can only come through people. Under the "captcha" You probably meant only to test the character recognition. Then, there are alternatives:
1. The speech recognition.
2. Recognition of graphic image, counting the number of similar objects.
3. The answer to the riddle, mathematics, to continue stable expression.

On their sites using the question when registering, which is chosen randomly from a list of questions, and in addition a great captcha. Bots have 3 months not seen.

counting the number of similar objects.

:) Exactly... Given three numbered images, the task is to find Lesny... Only numbers, not one, two, three, or something more serious and each captcha is different... That does not break the randomized.

You can search the database of Proverbs and sayings, in one of the words to arrange the stars and ask, what's the word. You can even list.

You can do SMS verification, if You have a little ReCaptcha.