Torque 3D looks to be challenging Unity for the 3d game in a plugin market; check out this feature on their home page:

Deploy any Torque 3D project from the World Editor to a web browser in seconds with our web publishing options. Torque 3D supports all major browsers and operating systems, including IE7, FF3, OS X and Chrome. Games perform at 100% native speed, with no performance cost, completely in your browser.

Both solutions,  of course, require that the plugins that host the native client be widely deployed, which is the biggest challenge. The gap between something like Flash or Javascript, and something like this, is measured in the hundreds of millions of installs. Of course, what you get for the native renderers in the plugins is desktop quality graphics.

The push on the other side, of course, is to upgrade the graphics in a form basically native to the browser, so you don’t need a plugin at all, or if you do it’s one you already have (because you visited YouTube once).