In December 2016 Microsoft promised that they would open up Cortana Skills Kit for developers and now they have done just that. By doing this Microsoft’s AI has opened itself up to numerous new custom abilities.
Microsoft has made it easier to get started and have stated that developers can simply reuse a lot of the code that they've created for Alexa or the Microsoft bot framework. These skills will work on any device Cortana is available on including new Cortana devices as well as windows 10, iOS and Andriod. It also works on standalone devices based on the Cortana Devices SDK.
The Cortana Skills Kit lets you:
Leverage bots created with the Microsoft Bot Framework and publish them to Cortana as a new skill
Integrate their web services as skills and re-purpose code from their existing Alexa skills to create Cortana skills
Connect users to skills when users ask, and proactively present skills to users in the appropriate context
Personalize their users' experiences by leveraging Cortana’s understanding of users' preferences and context, based on user permission
With 141 million people using Cortana developers will certainly reach plenty of users. And with half a billion people using Windows 10 that number is now sure to be on the rise.