Osmos HD, by Hemisphere Games, made its jump to the Android Market in late January, and brought with it some insanely relaxing gameplay. It was previously available on computer and iDevices, so you may have played it before it became available on Android. However, if you haven't played it already, the concept is ridiculously simple, but oh-so awesome.
Basically, you control a tiny little lifeform called a 'mote', and aim to consume or avoid being consumed by other 'motes.' It may sound odd, and way too simple to be fun, but that's just not the case.