One of the things announced during Google I/O 2014 was the Android L Developer Preview. It is an early access of the upcoming new version of Android, which is targeted for developers who want to make their apps ready when it launches soon.
The new features of the Android L Developer Preview include Material Design, the new default runtime ART (Android Runtime) to replace Dalvik, Enhanced Notifications, Project Volta (battery and energy efficiency), and a lot of new APIs.
To try Android L Developer Preview, go to http://developer.android.com/preview/setup-sdk.html. For now, only Nexus 5 and Nexus 7 2013 edition has available system images.
If you encounter bugs or have some feedback, you can summit it in the issue tracker at https://code.google.com/p/android-developer-preview/. You can also join the Android L Developer Preview Google+ community.