Filler is a great game by SimianLogic where you attempt to fill 2/3 of the screen with filler balls. The longer you press down, the bigger your filler ball gets; it stops growing when it either touches another filler ball, or gets hit by a bouncing ball.

Great fun, and almost instantaneously intuitive. For those that remember a super old school arcade game called "Quix," this is sort of along those lines - but with less lines, and more round balls. [via]