If you have been following the standards battle between rival mobile operating systems, you will have noticed that one tie-in that the sponsors of each mobile OS touts is the number of apps that are available on their respective platform. This, along with complementary hardware, is an indicator of the size of the ecosystem, which is often a key determinant of a potential user's adoption decision (there are clearly others).
It should not come as a surprise that some observers, often advocates of those mobile OS's with the quantitatively smaller ecosystem, argue that the total number is less important that the number of useful apps. To that end the folks over at AppBrain used the data provided by their system (a subset of Android users) to plot the popularity of apps on the Android platform as measured by the number installed. The resulting curve looks perilously like a power law distribution ... as it should if you follow Chris Anderson's argument in the Long Tail.