Please share interesting and useful apps for your mobile here (iOS or Android)

There is a whole world of apps so useful that we don't even know of and it would be interesting if we shared song amongst ourselves. A friend of mine introduced me yesterday to True Caller:

Identify and block spammers, search for unknown numbers, call friends and much more! With a community-based spam list from over 250 million users, Truecaller is the only phone app you’ll ever need.