A number of recent reports and analyses have been published in the past week indicating that the strength and growth in the mobile online marketplace is greater than anticipated. Only three years old, the app marketplace has been nearly doubling in size annually.

The size and rate of growth figures for the apps marketplace are frequently revised upward. The latest we’ve heard is from research firm Canalys which reports that 2011 app sales are expected to reach $7.3 million, climbing to $14.1 billion next year and $36.7 billion by 2015.

These projections may appear steep on first blush, but the incoming data is similarly remarkable. Apple announced this month that its App Store recorded its 15 billionth app download. The Google Android Market in May topped 4.5 billion.

These figures will continue to grow as more mobile users convert to smartphones that can run apps. Already the devices account for a quarter of all U.S. mobile phones, and just recently passed the threshold of 50% for new phone purchases.

As smartphone adoption grows globally, the opportunities for app developers will continue to grow. That’s because mobile accounts for 2% of the entire global GDP. The size of the mobile industry is growing at such a rate that it will soon match the amount the world spends on food.

These are very good times for software developers writing apps. The emergence of the app market is providing great opportunities for small business software developers to provide customer-facing products – and get paid for it. Apple alone has accounted for $2.5 billion in income for app developers and with the emergence of Windows Phone 7 and the continued growth of the Android Market, better days are still to come.