Google has finally pushed an official Android Auto app to the Play Store that will allow Lollipop phones to work with the first few head units and cars with support for the platform. It only works on phones running 5.0 or higher, and is pretty much useless without a compatible Android Auto system—you probably don't have one of those.
The first head units started shipping a few days ago, but it wasn't clear at the time if Android Auto would work. We reached out to Pioneer and were told that, no, Android Auto would not work until Google was ready. This app is apparently what we were waiting for to make the magic happen.
You might also notice the package name for the Auto app is Gearhead, which is the internal name for the platform we reported before the announcement last year. It will probably be a while before Android Auto is widely available enough for you to make use of this app, but at least it's there. Pioneer is the first with available head units, but more are expected later this year.
We should have an Android Auto system to test soon, so look for a hands-on post to go up at some point. If you want to install the app on a non-supported device for some reason, it's on APK Mirror.