In June, Steve Jobs said app makers had made $1 billion from iOS apps since the launch of the Apple App Store. But that number is expected to jump exponentially over the next year for developers on all mobile platforms.

Three years ago, we didn't know what a mobile app store was. Or how popular the concept would become in a very short period of time.

Since Apple's App Store opened in July 2008, however, mobile app sales have skyrocketed as app stores have popped up everywhere. Is it just a passing trend Research firm Gartner says no way.

In a forecast of what's to come, Gartner released a study today that says more than 17.7 billion apps will be downloaded in this year alone, up over 100 percent from the 8.2 billion downloaded in 2010.

And that's despite 8 out of every 10 apps downloaded being free, according to Gartner's findings. The money is rolling in for app makers, and the app store owners that they share revenue with. Gartner says by the end of this year revenue from mobile app stores worldwide will pass $15.1 billion--and that includes money made from smartphone or tablet users buying apps, and money made from ads placed inside those applications. It's a sizable jump from the $5.2 billion in revenue generated during 2010.

Every major mobile OS maker has an app store these days. But even some handset makers and carriers are getting in on the action. The list of app stores include: Blackberry App World, Android Market, Microsoft Marketplace, HP's WebOS store, Nokia's Ovi Store, Verizon, Samsung, and the one that started the craze, Apple's App Store.

The number of devices those app stores are available on is expected to increase. There are many people who still don't have a smartphone, but plan on buying one, and shipments of mobile media tablets are expected to explode by the end of this year. Apple has sold 14 million iPads already that have access to its App Store, and its competitors are expected to follow suit. RIM and HP are planning tablets with access to mobile app stores this year. And while there are a few Android tablets already for sale from Dell and Samsung, there are many, many more expected to start shipping over the course of the year.