Thanks to a generous App of the Week promotion, you can save a few bucks on Pivvot right now. It's completely free for iPhone, iPad, and iPod touch owners.
The game is all about "strategic avoidance" -- you maneuver a ball around obstacles as it winds through twisting paths by tapping on the screen. It's a straightforward premise, but the execution is spot on and the challenge becomes immense. If words like "I highly recommend this, especially since it won't cost you a cent" aren't enough, go read my full review.