Google announced an impressive achievement for its mobile OS today – the ten billionth app download in its Android Market.

While the milestone is impressive enough on its own, it is the rate of growth which is truly staggering – four billion of the downloads have taken place in the last six months alone. See the graphic below.

To celebrate the accomplishment, Google put a number of popular apps and games on sale in the market for only ten pennies, so if you are an Android OS user, go ahead and start shopping!