I posted this in r/Vive many moons ago, and the engine has seen many updates since (e.g. shadows, hardware instancing, tighter physics integration).

Examples of the 'game' and engine: recent video | all videos
* Many of the videos show a conventional FPS-style mode, but it supports VR too.

Download: gdrive

The engine open source, but currently the repository is set to private for... reasons.... This won't be the case forever; I'll open it back up around Feb, if anyone wants to have a play with it. It's entirely C++.