I remember the days of playing Bubble Popper on Pocket PCs, now I'm playing games like Angry Birds and even first-person shooters from my phone. Who would have thought that applications would have come this far? Some applications are so useful, I prefer to use my phone or iPad over my computer to take care of some laborious tasks.

One of the better things to come from this application explosion is a large chunk of money. A projection from Citibank states that App Store's sales will return $2 billion in revenue during 2011 alone. At first, this seems just too crazy to be true. Then again, I went on a binge last night and bought $20's worth of applications for my iPad. Not everyone goes on app buying rampages like I do (I can't help it sometimes), but with all of the iPods, iPhones, and iPads out there and all of the gift cards given for holidays, $2 billion begins to look more realistic, maybe even a low-ball.

Of the two predictions from Citibank, I would consider the revenue estimate for the App Store the easiest to believe. Citibank's other prediction is that global revenue will hit $27 billion by the end of 2013. That's right, $27 billion. It's hard enough believe that global sales revenue reached $4 billion just this year, let alone that it's expected to grow nearly sevenfold over the next two years. Even when you consider Android Market, Palm Catalog, Windows Marketplace, App World, and App Store all together, that number still seems a little wild. But with the way things have been growing lately, I'm open to believe just about anything when it comes to mobile applications stores.

We've seen Windows Marketplace jump from nothing to 4,000 applications in a matter of months, Android Market break 200,000 applications, and Apple's App store easily surpass 300,000 applications.  Those are all some impressive feats and I don't think any of them are anywhere near slowing down. If all of the current platforms grow as they're expected to and their application stores keep up this extremely rapid growth over the next two years, $27 billion can begin to look somewhat believable – yet still an unfathomable number.

Apple started an application revolution and I doubt they imagined stores composed of social networking and fart apps would return billions of dollars of revenue. It looks like mobile application development will be the side-field of choice the next few years. Who's going to become or befriend a developer now? I know I plan to try some coding on the side in the future so I can get my piece of that $4 billion pie.