I've been looking at a couple of options for implementing 3D interfaces in a web-browser environment.

Nothing native

The major browsers don't support 3D out of the box. They require a plugin. The least install-heavy is Java3D, which typically pops out of the browser using Java Web Start. The chances are your machine already has Java 5 or 6 installed. Samples.

Plugin options

Once you get past having to install a dedicated plugin, there are some good options. Google have released their O3D API. Samples.

One of the problems in rolling out 3D is that the client machine performance comes into play. Most of the Internet experience now is determined by connection bandwidth, but 3D interfaces and to a lesser extent client-side data processing place a much higher load on the user's machine. 3D typically requires dedicated hardware on the Graphics card (such as the shader engine of Nvidia's GeForce range e.g. 8800GTS).