http://random.irb.hr/signup.php This is probably the best CAPTCHA I've seen yet. It requires you to answer a Calculus problem, but don't worry, the answer will always be a real number. (And for the record, I solved one of the CAPTCHAs and registered. The problems aren't hard if you remember your Calculus.)
Wow...I think I've more or less forgotten about Calculus. There's no way I could solve that one. Good thing is that, it ensures only the right person qualifies to join
"Congratulations! You have successfully registered for QRBG Service." Now...what did I just sign up to?
I would never be able to sign up for this, no matter how hard i tried, i got lost at the first bracket