Mixed Reality Toolkit (MRTK) is an open source project that consists of set of components and features to help developers easily create cross-platform MR app development using Unity. It also allows developers to easily prototype via in-editor simulation that allows you to see changes immediately. Apps built using MRTK can run on HoloLens, Windows Mixed Reality headsets and OpenVR headsets (HTC Vive / Oculus Rift).
Microsoft yesterday released a major update for MRTK. The new MRTK 2.0.0 release comes with several new features, you can check out the video below to get an overview of them.
If you want to get started with MRTK 2.0.0 development, check out the hand interaction examples scene. It shows a wide variety of UX controls and interactions that are supported in MRTK.
You can download MRTK v2 here on GitHub.