SceneJS
NOTE: This project is now archived and no longer maintained. For a production-proven WebGL engine, try http://xeogl.org instead, which is actively developed.
SceneJS is an open-source 3D engine on WebGL that is geared towards rendering large numbers of individually pickable and articulated objects as required for high-detail engineering and data visualisation.
On the inside, SceneJS is a very fast renderer that uses performance tricks such as state sorting, shared node state cores, texture atlases and VBO sharing, while on the outside it has a declarative JSON-based API that's convenient to hook into the rest of the application stack.
http://scenejs.org
Read the chapter on SceneJS in OpenGL Insights:
http://www.amazon.com/OpenGL-Insights-Patrick-Cozzi/dp/1439893764
Try the BioDigital Human, a medical visualisation platform which uses SceneJS for 3D renderering:
http://biodigitalhuman.com