WebGL is a binding of OpenGL ES 2.0 for Javascript. It lets webpages do fast 3D graphics in the browser. The spec is still a work-in-progress, and support is only starting to appear in browsers. None of the regular releases support it yet, so you may need a nightly build.
Firefox nightly builds support WebGL. You can download those here:
http://www.mozilla.org/developer/#builds
However, you need to turn on WebGL manually in the configuration.
Type about:config in the address bar, then search for "webgl"
and change the value of webgl.enabled_for_all_sites to true.
WebKit nightly builds support WebGL. You can download those here:
Nightly Chrome builds have support for WebGL, but it needs to be enabled through a command line switch, and also needs the sandbox to be turned off through another command line switch.
Opera does not seem to support WebGL at the moment.