Odds are if you have a mobile device, you've heard of Fruit Ninja. It's pretty much perpetually in the top fifty rankings, and it even had an Xbox 360 release.
The simple concept of slicing fruit with your fingers has been copied many times, but none of them can match the original. If you somehow completely missed this fun little game, you can pick it up right now for free off iTunes for the next 24 hours (until 6:00 AM GMT Tuesday).
Other Halfbrick games will be on sale as well, including Monster Dash and Age of Zombies. You can find the full listing here, but my absolute must-have is Jetpack Joyride -- which has been free for quite sometime anyways.