Developers can download and create skills for Alexa-enabled devices with Amazon's Smart Home Skill API, included the Alexa Skills Kit. Amazon says:
"Less than a year ago we released the Alexa Skills Kit, making it possible for any developer to create voice experiences for Alexa," said Rob Pulciani, Director, Amazon Alexa. "Today, we have a vibrant community of tens of thousands of developers who are learning about the service, bringing useful and innovative skills to every aspect of Alexa customers' lives, and introducing their own users to the magic and simplicity of hands-free, voice-driven interactions. We're excited about the 1,000 skills that are already available, and can't wait to see what developers create with the next ten thousand."
Some of the companies that have created skills for Alexa-based devices include Capital One, Domino's, Kayak, Uber, Fitbit and more.