It has since been taken off the Google Play Store in accordance with their updated policies, but I used to use an app called Crypto Miner by Jesus Oliver. That's legit -- actually mines... albeit very, VERY slowly, as everyone else in the thread has already noted.

It won't even make you enough at a pool to hit minimum withdrawal. I doubt you could make enough mining any popular altcoins either.

Maybe if you had a thousand phones. Or ten thousand.