Three.js is an open source javascript library for drawing 3d graphics on the web. It was created by Ricardo Cabello (aka. Mr. doob). It grew out of the work he had done for Flash on the Papervision 3D library. Originally it was created as a renderer agnostic API with a 2D Canvas and SVG Renderer. As WebGL became available in browsers a WebGL renderer was added. This helped grow the popularity of both Three.js and WebGL because it made WebGL programming more accessible to developers.

To get Three.js you can visit threejs.org which has a grid of examples of the library used in production sites. It also contains links to download the library, visit it's github page, documentation, and a bunch of great examples.