You can re-live the growth of the mobile app store thanks to this graph produced by ShoutEm. It traces the incredible growth in apps available for iOS as the App Store grew from 100,000 apps after its first year to the current 500,000. The Android Market story can be followed from those initial 50 apps to the recent count last summer of 250,000 with 6 billion downloads. Other stores like the Windows Marketplace are also included in the timeline.
The timeline ends in August of last year, so the figures are not the most up to date, but they do present a historical, accurate picture of the exciting growth in the industry.