A few days ago Omni Rom announced that they would be adding hotwords to launch Google Now to their rom. They also announced that this would be customizeable to open up an assortment of other apps. While this is awesome it would require you to run their rom. Developer "mohammadag" has released an xposed module that allows you to add the "OK Google" hotword to launch Google Now on any launcher. Installing this module is super easy. Obviously you will need to be rooted and have the xposed framework installed. Then simply install the module and reboot your phone. The developer also decided to open source his code so you may see this feature pop up in an update for your favorite launcher soon. More info and the download can be found at the link below.
via xda