A while back for a Superhero RP on the forums I came up with a potentially game-breaking superpower: the ability to manipulate kinetic energy.
In a nutshell this character could control kinetic energy by influencing how much something speeds up, slows down, comes to a complete stop, etc. Basically she was able to, in one way or another, control anything that moves. She could turn pebbles into deadly projectiles, slow down and stop bullets in mid-air, run incredibly fast, jump off of a building and slow the speed of her descent so she could land safely, and so on. This power had so many potential uses that I had to very carefully explain the science behind most of her actions so it wouldn't be considered borderline "God Modding".