In 2014 Apple introduced Swift, a programming language designed to simplify making apps for iOS and OSX. In June of this year they promised to make the source code available not only allowing developers to submit improvements and fixes to the language but open the door to cross platform compilation of apps written in Swift. They've made good on their promise.
Development of cross platform compilers isn't being done by Apple but the open source compiler and standard library mean that developers can already target linux. Commonly, iOS and OSX development is done in Objective-C but a number of high profile apps are now written in Swift including LinkedIn's own app.
The open source version of Swift will not be seeing its way onto the app store though, Apple retain tight controls to ensure that apps are safe for their users. Apple's version of Swift will remain a separate entity still tied to XCode.
Swift became one of the fastest growing languages after its launch, With talk in May of this year about Microsoft looking at Swift could Apple have the answer to their desire to get more apps on the Windows Phone? If you're looking to build an app though the way to go is still individual development for each platform, it is doubtful that will change soon.
If you are looking to develop a Native App or are interested in an app that utilises Swift get in touch.