Does it look familiar to you? Yes, the "plus icon" from Google+ is already out here.
However, more apps are on the way! We expect Gmail, Calendar and some more system apps to be updated with this new, redefined design.
This new design is called Quantum Paper and it's going to be available not only for Android platform but also on iOS and web. Remember this screenshot?
We'll probably see this design with upcoming app updates and/or system updates. Nothing is confirmed, though, and we can currently only speculate about further information.