LÖVR projects work on Windows, macOS, and Linux. You can run the same code on an HTC Vive or an Oculus Rift, and there's even a drag and drop tool for exporting projects to WebVR.
LÖVR is minimal, clean, and really easy to pick up. It's scripted with Lua, a fun little scripting language. Most common tasks are reduced to just a few lines of code, and everything is designed for VR.
LÖVR is open source software, meaning it's 100% free to use and doesn't require accounts, paid licenses, or royalties. The code can be found on GitHub and is released under the MIT license.