Google announced yesterday at Google I/O a neat expansion of Google Now that will be built into Android M, the upcoming release of Android. It is called Now On Tap, which basically means what it says, you tap and Google Now appears and shows you contextually relevant information, to you, based on the content you are looking at.
The key point is that this works through App Indexing, so if you are not yet doing App Indexing, it doesn't only help with ranking your app content in Google mobile search but also helps drive native Android Google Now related (non searches) to your app's content.
There have been many third party services that have been going into this area but Google simply wiped them out with this release.
Here is how it works:
If a friend emails you about seeing the new movie Tomorrowland, you can invoke Google Now without leaving your app, to quickly see the ratings, watch a trailer, or even buy tickets—then get right back to what you were doing.
If you’re chatting with a friend about where to get dinner, Google can bring you quick info about the place your friend recommends. You’ll also see other apps on your phone, like OpenTable or Yelp, so you can easily make a reservation, read reviews or check out the menu.
There are many more examples at the Google blog.
Forum discussion at Google+.