Amazon Echo: What can Alexa do and what services are compatible?

Amazon Echo is a hands-free speaker controlled with your voice. It features a personal assistant called Alexa, who will perform various tasks for you and control various systems. There are seven microphones within Echo, all of which feature enhanced noise cancellation and far-field voice recognition, meaning you can ask Alexa a question from any direction, even when playing music, and she should still hear you. Amazon Echo's personal assistant will respond to the wake word "Alexa". If you have more than one Echo, or Echo Dot in your home, Alexa will respond from the device closest to you.