Google Now on Tap
One of Android Marshmallow's most exciting features is one that hasn't been released into the Developer Preview.
Google Now on Tap, which can be seen here on video, is one of Android 6.0's most important features. When you hold down on your device's virtual home button, Google Now will read the contents of your screen and bring up cards that feature relevant information. It's pretty incredible.
For example. if you're talking to a friend in Google Hangouts about the San Francisco Giants game, you can hold it down and it will presumably bring up the score of a game in progress. Or if you're chatting about a movie star, it'll bring up that person's bio and the latest news.