At its WWDC keynote today, Apple confirmed that Siri would be coming to the Mac with macOS Sierra. Siri's icon will sit in the top menu bar next to the Notification Center toggle.
Siri can handle context-sensitive searches for things such as files, allowing users to search for files they worked on in the last week or just the files an individual co-worker sent a user. Siri for Mac, which can also work while a user is in a full-screen app, can search the web. Siri's web searches can also be pinned to the Notification Center.
Like Siri for iOS, Siri for Mac can also be used to bring up music, send text messages and open apps. Siri can also be used in conjunction with other new macOS Sierra features, like the updated Continuity and Apple Pay.