Mobile phone users will spend $6.2 billion downloading applications this year, up from $4.2 billion last year, according to a report from Gartner. And the trend appears to only be starting with Gartner forecasting sales in mobile application stores of about $29.5 billion by the end of 2013.

And that's just paid applications. The vast majority of downloads -- an estimated 82 percent -- are free to consumers.

“Application stores will be a core focus throughout 2010 for the mobile industry and applications themselves will help determine the winner among mobile devices platforms,” said Carolina Milanesi, research director at Gartner, in a release. “Consumers will have a wide choice of stores and will seek the ones that make it easy for them to discover applications they are interested in and make it easy to pay for them when they have to."

The battle for the mobile phone application ecosystem is a crucial one as Microsoft looks to catch up to Apple and Google.