OK, there are more options than the ones I listed in my last post. Some others include:
Developed by the Fraunhofer Institute in Germany, this is a "high-performance Mixed Reality (MR) system, which combines various components to provide a single and consistent interface for AR/VR developers." Runs on Windows with beta versions available for Linux and OS X. Full X3D with extensions for clustering, devices, stereo and more. No browser plugins (although they're on the roadmap). Not exactly sure what the licensing is like, but they seem to be free to use for non-commercial stuff at least. Static logos on scenes. No source available AFAICT.
This is probably a better solution than I give it credit for, but for various reasons it turns me off.
Papervision3D (or PV3D as it is known) is a 3D engine written in Flash. Away3D is a fork of PV3D.
From a purely technical point of view I'd much rather use an X3D plug-in that calls the underlying hardware-accelerated 3D directly. Obviously the performance would be far superior; also the language itself is much better suited to the task. But the reality is that I can't really do that with a free, cross-platform player that doesn't have someone else's logo in the corner. And requiring people to download a plug-in is still (seen as) a significant disincentive to new users. I think that installing a Firefox add-in might be less of a barrier, but there aren't any for VRML or X3D (yet).
However, in other ways it's not really that bad. Flash 10 is on its way and apparently it will include some 3D support for projects like PV3D. (See this interview with Rob Bateman of Away 3D and this interview with Ralph Hauwert of PV3D.)
Also, the sort of stuff I want to start with doesn't require anything terribly amazing in terms of sheer 3D horsepower. Hopefully by the time I have something that people actually find useful, PV3D and Flash will have evolved together into more of a fully-fledged Web 3D (if not X3D) solution.
In the meantime, I guess PV3D is a case of "worse is better", at least compared to X3D and Croquet.
See also the Papervision3D developers blog, some tutorials and other resources and the code.
Technically the best solution. Fully open-source and cross-platform. IMHO destined never to catch on widely (compared to Flash). Which just goes to show that there ain't no justice in this game.
A few links to vaguely relevant 3D stuff on the web (some courtesy of Miles):
Python Paste