We developed the viewer on xeokit, an open source WebGL-based 3D library from xeolabs.
The viewer is also available as a standalone BIM viewer, bundled as part of the xeokit SDK: https://github.com/xeokit/xeokit-bim-viewer
To use the viewer, download it from GitHub, or fork the repository, then drop in your own models, and you're good to go.
It's a lot of fun working with the OpenProject team on this software. The development process is methodical and steady, with great communication, using the OpenProject platform itself to coordinate tasks and keep things on target. We'll keep polishing this, and will continue to use the xeokit SDK to keep OpenProject on the cutting edge of browser-based BIM visualization.