Microsoft has been teasing us with "beta" versions of Cortana for both Android and iOS for a while. For folks wanting to try out the Redmond-forged digital assistant, the teasing is over for the first official version. Microsoft just announced the availability of Cortana for both Android and iOS. You can pick it up for FREE on either the Google Play Store or Apple's iTunes.
Microsoft also shared that Cortana will actually be integrated into the Cyanogen OS-powered OnePlus One some time in December (with the 12.1.1 Cyanogen update). Keep in mind that Cortana on anything but Microsoft will be pretty "stripped down," (try not to visualize that too much). It will include features like track reminders and notifications for flights and packages across all platforms. You will also be able to manually ask Cortana questions on all the platforms.
Other specific features will be different from platform to platform. For example, on iOS you will NOT be able to use the "Hey Cortana" voice command features, but that will work on Android and Cyanogen. The problem is that Apple is not willing to open up iOS enough to make that feasible.
Cortana apps will initially show up in US and China at launch, but will eventually come to other areas of the globe. Cortana will require your device to run iOS 8 (or above), or Android 4.1.2 (or above).
Here are links if you want to check it out: