The Amazon Echo is a pretty neat little device, but at $US180 ($245) it's also pretty pricey. If you want to make your own, YouTube user Novaspirit Tech walks you through the process of how to do so with a Raspberry Pi.
To build this DIY Echo, you'll need a Raspberry Pi, USB sound card, external speaker, and a push button. That push button is also the catch here. Similar to the Amazon Tap, the Pi version requires that you push a button before you can initiate voice commands. It's a caveat that might turn many people off, but it's still a pretty cool project anyway, and once you have it set up there are plenty of possibilities for tweaking it to your liking.
Head over to the YouTube page for all the links you'll need to set this up one your own.