Microsoft is rolling out Cortana, its personal digital assistant, on iOS and Android devices starting today, December 9.
Microsoft has been testing publicly its Cortana app for Android in the U.S. since August, and privately testing the Cortana app for iOS in the U.S. and China since November.
Microsoft officials have said that the Cortana apps for iOS and Android won't be as feature-rich as Cortana on Windows and Windows Phone, as it won't be as tightly integrated with the operating system. For example, on iOS, the "Hey Cortana" wake-up feature won't be available, at least not initially.
Microsoft also is integrating Cortana into Xbox One, and is expected to make Cortana available as an integrated component of the coming Cyanogen Android distribution.
The Cortana app for iOS is available as a free download from the Apple app store (thanks for the link @NotCassim). The Cortana app for Android also is available as a free download from the Google Play Store.
Update: Here's the Microsoft blog post announcing availability of Cortana on iOS and Android. In it, officials say the Cortana app is available today in the U.S. and China for iPhones on iOS8 and up and Android 4.1.2. It will be available in the U.S. on OnePlus One powered by Cyanogen OS with the 12.1.1 over-the-air update later in December. The Cortana app will be available on more Cyanogen OS-powered devices next year.