Most Captchas aim to separate humans from spambots, but a few creative Captchas floating on the Web probably aim to separate the super-humans from the mere mortals. Tongue-in-cheek aside, I'd definitely like to see more topological Captchas like the one above.

Geek Technica has a neat list of 10 such Captcas that have gone a bit too far in proving that you're human: Link