Backdoor (free), which works with Facebook and Google+ sign-in, is a social-messaging app created by Daniel Singer, the teenage co-founder of YouTell. YouTell is an anonymous web service that allows you to invite friends to comment, without revealing their identity, on questions you present to them. The idea is that they can answer your questions truthfully while remaining anonymous.
Backdoor provides a similarly anonymous messaging service that your friends can use to text you. The hook is the unknown identity of your texters, with in-app purchases required to unlock clues to their true identity.