Anything but Recaptcha
But despite that I don't like it, I still manage to use it since it helped me block some spam users from signing up and sending comments to my website with the help of akismet anti-spam plugin.
But anyway that's a little guessing game to ensure your safety as well. And I guess it would be annoying if something else should be guessed.
Now I'm just kidding about the complex math problem, although there is also a basic math captha being used by other sites.
The thing is, we wouldn't be doing this if it wasn't for spammers. Let's just hope someday we could find a better alternative in dealing with internet security.