Thunk

A lot of Web Applications nowadays which has some form of a registration feature (like forums), employs some form of a turing test (or rather, a reverse turing test), like CAPTCHA, to subvert automated registration.
Though it’s a good measure, it’s not entirely bullet-proof as some registration bots can still manage to recognize to distorted image.

One interesting registration systems is the service registration for the Quantum Random Bit Generation System: http://random.irb.hr/ here’s a screenshot of their registration, pay attention to the bottom part:

Funky reverse turing test of random.irb.hr

It’s actually called a MAPTCHA (Math CAPTCHA). Though it’s complex, I think it’s proper since the ones who are going to be using their services are Mathematicians or Scientists or at least those with good math background. In other words, they targeted their turing test to the people who are likely to use their services… I won’t say that every registration system should use that though… unless they want to drive people away.