Apple is closing in on 10 billion apps downloaded via the App Store -- complete with a $10,000 iTunes gift card for the lucky user who makes the milestone-setting download -- and in the process is revealing which apps have done the most to help reach that figure. Music apps figure prominently in the results.

Among the top all-time paid iPhone apps, various installments of the music rhythm game Tap Tap Revenge claim three spots: Tap Tap Revenge 3 at No. 2, Tap Tap Revenge 2.6 (?) at No. 5, and Tap Tap Revenge Classic at No. 8. Among the top free iPhone apps of all time are Pandora at No. 2 and Shazam at No. 4. And among free iPad apps, Pandora takes the top spot at No. 1.

And according to a recent report by MarketsandMarkets, the global app market will be worth $25 billion by 2015, from $6.8 billion last year. And while Apple will command about 20% of that figure, in terms of volume, the company won't necessarily be the leading app store by that time. MarketsandMarkets predicts independent app stores like GetJar, which offer apps for multiple smartphone platforms, will outpace native app stores in terms of downloads by 2015.

Overall, the growth of mobile apps is staggering. MarketsandMarkets says mobile app downloads from 1 billion downloads in 2008 to 6.4 billion in 2009, with North America responsible for 41.6% of revenues.