This app solves a puzzle that was posed by a billboard in Silicon Valley in May 2013.
The billboard is shown below (it was at the site of the old Circle Star auditorium in San Mateo).
The puzzle is basically find the largest 15 digit number that is both
1. a perfect square
2. a palindrome
Then plug that number into a url to go and look at the recruitment possibilities of a company called rocketfuel.com (cool name). The app renders a haptic (vibration) effect for every half million numbers it evaluates, so you can track progress towards a solution.