One of the few indie titles that Microsoft paraded out in front of a captive E3 audience was Below from Capybara (Superbrothers: Sword & Sworcery EP). Although published by Microsoft, it will not be exclusive to Xbox One.
Nathan Vella & Kris Piotrowski issued a statement on Capybara's website, noting that Below will arrive on Microsoft's console first, but will also expand to other platforms after a brief period of exclusivity. "Like many games on stage at E3, Below will be available on the Xbox One first. Then, after a period of time, CAPY can bring it elsewhere. It’s always been this way."
Specific platforms are not named, though the latest issue of Game Informer does mention PC and PS4.