I think questions like « Prove that the union of finitely many compact sets in a topological space (X,τ) is also compact » would certainly achieve rate limiting and boost global math skills.

I seem passing by this really funny captcha on the Quantum Random Bit Generator Service's [1] signup page.

[1] http://random.irb.hr/signup.php

That's good, but unfortunately security through obscurity (in the true sense of the word).