The K3D engine ("Kevs 3D", no relation to the k-3d library of similar name) written entirely in JavaScript and HTML5 canvas. All 3D maths is hand cranked and rendering performed using the nifty Canvas APIs - no plugins or extensions required.
The code for the simple-but-fast 3D pipeline was originally written in C/C++ back in the coding dark-ages of 1996, converted to a very-nifty-for-the-time-thanks Java 1.0 Applet in 1998 (the first 3D in Java that I know of! and well before the Java3D APIs existed) and recently converted to modern JavaScript and HTML5 Canvas.
Runs on Chome, Firefox, Safari, Opera and IE9 browsers. Chrome is the fastest!
Objects can be displayed as points, wireframe, solid polygons, textured polygons and shaded using plain colour, depthcued (z-order) or light-sourced - things can be rotated around and moved about to make cool demos. A simple linear-additive lighting model with any number of coloured lights can be applied to solid objects.