There's OSVR, which is about as modular as you can get - there are plugins to interface with e.g. SteamVR, and it's compatible with VRPN for peripherals. (Very much a dev kit though, if you're looking for something you can just plug in and use the OSVR headsets are absolutely not that.)

I have to wonder if this team could have gotten more mileage out of working with OSVR, since a lot of the work to connect it with existing VR apps is already done. But there's certainly value in doing it all yourself!