Squareball is a weird one; it's probably best described as a platforming game, except that you move the platforms rather than the jumper. The main character is a little white ball (or square in this case -- the game has a retro blocky pixel aesthetic) that constantly floats from top to bottom on the screen, and you can slide (or tilt, according to whatever option you choose) the actual stage around the ball, moving it through various nooks and crannies while trying to "collect" colored blocks.
The concept is tough to explain, but once you see it in action, you'll understand right away. The goal is to move the stage at the right time in order to keep the ball bouncing around where you want it. It's actually really fun, despite the simplicity, and over the 20 various stages, there are some pretty clever puzzles. The game also has OpenFeint integration, and the 8-bit music is pretty catchy as well.
It's a good deal for only 99 cents. There's also a lite version to try out if you just want to see how it works. I can't see anyone really disliking the game, but it is fairly simple. Still, at just a buck, it's worth a purchase just to have it to pick up and play when you need a mindless little reflex game.