Previewed at WWDC, launching in the fall.
WebGL Support Begins to Arrive in WebKit
Wolfire speculates that if it gains widespread adoption, "full-fledged 3d video games" could becoming more common on the web. Like many of the other emerging web technologies, this could threaten to dislodge Adobe's Flash technology.
WebGL is not yet available in the nightly builds of Webkit and is currently only available if you compile the source yourself.