In November, Google announced custom voice actions for Google Now, which means you can integrate your app content directly into Google Now. So if you want, you can enable Google's "Okay Google" voice action with your apps voice actions to trigger it to work with your app.
Well, that feature launched yesterday afternoon for searchers.
Google said on Google+, "we launched our first set of partners for custom Google voice actions on Android."
Google added:
This feature will enable people to say things like “Ok Google, listen to NPR” or “Ok Google, show attractions near me on TripAdvisor.” We’re currently piloting custom voice actions with a select group of partners, but we plan to open it up more widely in the future.
For this to work, the searcher needs the latest version of the Google app and the Android apps.
If you have an app you want included, you need to fill out this form.
Forum discussion at Google+.