One of my favorite features on the Moto X is "touchless control": even if the screen is locked, or you are in an app, you can say "OK Google" and wake the phone up for a search. Now, thanks to an update to the Google Search app, any Android phone can now trigger a voice search with Google's hotword.
The feature, which was spotted by Droid Life, unfortunately doesn't work when the screen is off. The Moto X has that ability, but it also has unique low-power hardware specifically for the feature.
Voice search on any screen is opt-in for now. You'll find the option to turn on hotword detection in the Google Now app: scroll to the very bottom, and tap the Menu button. Options are in the voice settings menu. You'll also find an option to opt out of Voice History, a new feature which presumably tracks things you've said previously in order to get better at recognizing your voice.