It might be cheating to include Google's own apps in this list, but Google's Inbox is one of the best examples of Material Design in action to date, so I'd be remiss to ignore it. Inbox has everything you want from a Material Design app: bold colors, fun pull-to-refresh actions, easy one-handed navigation, and informative animations. It actually makes it fun to manage your inbox, which is no small feat. Inbox is free and requires an invite to access, though those are fairly easy to come by. Google says that support for Google Apps accounts is in the works, but for now, it's Gmail accounts only.