Android has supported live wallpapers since version 2.1, but most of the apps taking advantage of it are a little too busy for long term use. Muzei is a different take on the Android live wallpaper.
This app takes various images and puts them behind a blur effect to create an understated, but very attractive backdrop.
Muzei comes from Googler and part-time developer Roman Nurik, who also created the DashClock widget. Muzei includes support for a daily piece of art that will be used as the background, as well as local content on your device.
The Gaussian blur effect keeps the background from overwhelming the icons and widgets on your home screen, but you can see the unblurred image by double-tapping on the screen at any time. Alternatively, open the Muzei app to see the regular image in immersive mode (Android 4.4+).
Nurik chose to include a developer API for Muzei that allows additional image sources to be grafted on.
A quick search in Google Play shows several great extensions already available just days after Muzei's release. You can pull in images from Reddit, 500px, Tumblr, and more. Just install the extension app and select the source in Muzei.
Most of the extensions include options to control the refresh rate and restrict mobile data usage. Muzei itself has settings to control the strength of the blur and dim effects. Muzei doesn't seem to cause any lag on devices and uses no appreciable amount of battery life. Best of all, it's completely free.