These are listed in no particular order, and vary a bit in relevance, but I consider them helpful, or interesting, and at least a glance. If in doubt, have a look at the annotation further down.
Mesa is a 3-D graphics library with an API which is very similar to that of OpenGL. Mesa has been under development since 1993, and is written by Brian Paul. I consider this a very important project, as Mesa provides a GLPL copyrighted source that works with Unix, namely Linux, and allows development with commercial OpenGL in mind, as is available with Windows NT. Even while OpenGL might be slow in some implementations, and will not be supported with low end graphics accelerator boards, it's still the best choice for cross plattform development.
There's a good chance we'll see a GLINT 300SX version of Mesa in about two months, with information provided by 3Dlabs. As far as speed is an issue, there's always the possibility to take the Mesa source and introduce a completely separate code path for your particular functionality, while keeping the API. If you're interested to support a relevant free 3D graphics library, Mesa is the perfect choice.
In the aftermath of the fairly ridiculous LZW patent/Unisys licensing issues, the need for a portable but legally unemcumbered file format led to the creation of PNG. This again is a very important achievement for a free community. PNG is supposed to replace GIF files, and will do, as there is already web browser support for inlined PNG, and the specified lossless compression is a lot better anyway.
Any project with multi-server and Internet use in mind will have to incorporate mechanisms to handle links, obtain resources, and negotiate connections. The W3C reference sources provide an implementation of the standard for noncommercial use, including simple example servers, robots, clients.
The Virtual Reality Modeling Language was supposed to be a free standard for interactive virtual reality on the web. The first specification was based on ideas from SGI's OpenInventor, and dsefined a scene description file format. SGI released the source of the qvlib parser, which is available for any project in several releases at the VRML repository. You will find a lot of other resources there, too, including objects, textures, and links to drafts on distributed VR and behavior issues.
Unfortunately, there is only one source release of an actual VRML browser, which is VRweb released by the Hyper-G group, and based on Mesa. If you have got use of VRML via the web in mind, you should have a look at VRweb.
Inspired by Chris Laurel's "wt" project, Phillip Stevens started WebView3D as a spare time effort. It is currently supported by IBM as a paid research project, and the upcoming release will include networking support and other stuff in addition to the DOOM-style renderer. The sources are available for noncommercial use. Among the many DOOM offsprings, WebView3D is, too my knowledge, the most advanced, except for VPE, which is not available as source.
The POVRAY project by Chris Cason and the POVRAY team gave the community a raytracer that has been put to professional use more than once. In many aspects, the success of POVRAY is what I want to accomplish with Difference Engine during the upcoming years. POVRAY represents a really good solution to the problem of getting some reward out of your work, while sticking to the ideas of freely available software and cooperation.