Have you ever been jealous of Cortana on Windows 10? Don't be anymore, Siri is coming to the next version of OS X.
OS X 10.12, codenamed 'Fuji' internally, is going to feature Siri as one of it's massive features. Rumors point to Siri being located in the status bar, and as you hit it, it will drop down.
OS X 10.12 will be the successor to the current release of OS X, El Capitan, and will be announced alongside iOS 10. Both iOS 10 and OS X 10.12 will be announced in June, at WWDC 2016.