Android: Google Now on Tap was easily the coolest feature announced at Google I/O this year. However, you still can't use it yet. If you want to get an idea of how it works though, Microsoft has gone ahead and added the feature to its Bing app.
The feature allows you to scan whatever page you're currently reading and get search results and or links to other apps (like IMDb or Wikipedia) to take action. In my brief tests the feature seemed a little rough around the edges. Some contexts that seemed like they should have at least been able to pick up some proper nouns didn't return any results. Microsoft has also announced that they're making an API available to developers, but it's not clear how many third-party devs are actually plugging into the feature to improve it. While there's still room to improve, this one is actually available, while Now on Tap won't arrive until Android Marshmallow finally drops.