Three.js
is a lightweight javascript library to create 3D visualisations (using WebGL, SVG, Canvas or another renderer).
Clara.io
is an online 3D modelling tool using WebGL.
Famo.us
is a javascript rendering engine to create web-apps with 3D user interfaces that promises to run smooth (60FPS) on mobile devices.
Famo.us claims to solve the HTML5 performance issue by avoiding DOM-layout and repaints, which are very expensive computations. Instead, Famo.us
keeps a flat DOM and uses CSS3 transforms for all layout and positioning. Unlike the others you mentioned, Famo.us does not render using WebGL (yet), as Safari does not have this enabled by default.
Famo.us includes a physics engine to provide realistic and complex user interaction (e.g. bounce on a scrollview, 3D coverflow, that sorts of stuff). In Famous, you construct a scene-graph and add animations, events and physics to make everything come alive.