Now, this feature has been ported to another Android devices as well as an Android application in Google Play Store!
Open Mic+, lets you simply take control over your Android device by just saying "Okay Google". When you say this "command", the Google Voice Search will open on its own listening to your voice commands.
This feature may come very useful in car, for example, when your hands are busy.
Open Mic can launch Voice Search also by hovering your hand in a front of your device!
Unfortunately, the app is currently in development and may not work properly on some devices. It also drains battery little more than you may expect.
Well, try for yourself!