Google Now offers quite a few voice actions, but so far they’ve been strictly related to Google services like search, reminders, and control of various Google Now cards. But the Android Developers Google+ account announced new custom voice actions that opens up Google Now to third parties (through Google, of course).
The first custom voice actions announced are for two app partners, NPR and TripAdvisor. You can say “OK Google, listen to NPR” to launch the NPR app and start listening, or “Ok Google, show attractions near me on TripAdvisor” to open the TripAdvisor app. And of course, there are infinite possibilities for developers to implement custom voice actions.
Google is also asking developers to chime in about voice actions they’d like to implement, because despite the feature being exclusive to partners, Google expects to open it up later on. So if you’re a dev, hit the source link and contribute!