X3DOM (pronounced "x-freedom") is a free open source framework that integrates 3D graphics with HTML5. Prior to X3DOM, X3D scenes not only required browser plug-ins, but the 3D scene could not interact with HTML actions (button clicks) and other DOM-triggered events. X3DOM allows Web developers to manipulate 3D content simply by adding, removing, or changing DOM elements -- and the X3D language can be picked up in a weekend.
Most modern browsers already support X3DOM. The latest versions of IE, Firefox, Chrome, and Safari all have the necessary WebGL support and are X3DOM compliant. Sounds fun? Check out the great X3D language tutorial and the examples on the X3DOM site.