The Google Sound Search widget was previously available on the Nexus, but in its latest avatar, it can be installed across all Android devices. Once installed, add it to the homescreen and let the search widget identify any music playing around you. If you are running Android Jellybean, you can add the widget to the lockscreen. The Song Search widget also keeps a history of all identified songs.
You can sync it across all your Android devices running the widget and use the identified songs to purchase their digital copies from Google Play, and add them to your Google Play Music library.
So, just head to the Google Play Store and install Sound Search for Google Play. Next, go to the widget picker on your device and drag the Sound Search widget onto your home screen. Touch the widget to identify music playing around you.