MM2X Forum

Microsoft developed what was thought to be impossible, by developing a cross platform system for Xbox One. Such thing would allow players for either Xbox, PS and PC to play together. For now it just allows a upcoming game between Xbox One and Windows 10 - Rocket League - but it all depends of the interest of other companies like Sony and Valve.

Source: https://news.xbox.com/2016/03/14/letter- ... dates-gdc/

I believe Killer Instict will be a game that will also soon be cross platform. Microsoft also initially made it seem that there would be cross platform multiplayer between Forza 6 and the Windows version, Forza 6 Apex, but a leaked preview of the Apex game showed no multiplayer, so that was a bust.