Anyone that has ever used the internet has experienced the scourge that is spam bots. If you want to protect your favorite site from spammers, the folks at Croatia's Ruđer Bošković Institute have the answer. It is absolutely guaranteed to keep non-humans out . The site presents visitors with a bit of advanced math they must work out.
This isn't just a captcha, this requires real know how and the vast majority of humans (forget the bots) won't be able to do it. The site helpfully lets you know that if you can't make heads or tails of the problem, you can reload and "you'll (probably) get another, easier question." The science-types at Ruđer Bošković Institute seem pretty keen on keeping out the riffraff.
The LA Times does point out that Wolfram Alpha can solve the problem for you, but that still relies on you knowing how to input the problem correctly. Is there a good middle ground here?