No one knows how many Amazon Echo devices Amazon has sold, but the number is almost certain to grow now that Amazon is finally selling the always-listening "digital assistant" device in Best Buy, along with other retail outlets. Amazon Echo, which has been a surprise hit for the Seattle-based online sales giant, is an all-black, intelligent speaker that includes Amazon's Alexa digital assistant. The device's sensitive microphone array means that you can say "Alexa" from anywhere in the room and Echo will come to life and listen for a command or query. It's cloud-based, so Echo and Alexa tap into your Amazon account's content (music, books) as well as weather, news sources and more. In fact, Amazon is constantly adding new services (like the ability to control smart home devices like lights). The company is also adding new Echo siblings, recently unveiling Amazon Tap, a battery-powered version of Echo that only listens when you hit a button (it looks a lot like an Ultimate Ears speaker) and Amazon Dot, a smaller version of Echo that features Echo's microphones, Alexa's intelligence and connects to your existing speaker system. Both Amazon Echo and Tap (in late April) are coming to Best Buy and Lowe's (currently, you can only order Echo Dot through other Amazon devices). The hardware has been on sale at other retail outlets, including The Home Depot, Staples and Radio Shack, since November of last year. All Echos will, of course, also be available at Amazon.com. Have something to add to this story? Share it in the comments. TopicsAmazonAmazon AlexaAmazon EchoBONUS: Amazon Echo review