Silli is the app which enables you and your friends to continue to take each other by surprise, by allowing you to change your friends background image remotely and anonymously.
You simply snap an image, choose who's phone background you want to change and Silli will discreetly change the recipients background image without letting them know. Users are then able to earn points by correctly guessing which friend changed their background image – but you only get one guess each time!
via BetaList http://bitly.com/28SJ78s
by Robert Hughey (Google+)