We now have the full sources to two plugins on the CVS, and we plan on adding more soon.
3D Box is an interesting plugin because it demonstrates how to render 3D from a plugin. However, much of the code may be subject to change. The way it renders 3D is certainly sub-optimal; the engine may change in future to make 3D more efficient - so be aware of this if you plan your own 3D plugin. 3D Box's code is a little messy, but gets the job done and hopefully you can make sense of it (if you have any questions just ask). 3D Box was developed by David and I.
XAudio2 is a non-layout object like Mouse & Keyboard, which doesn't do any drawing. It provides a complete game audio engine. I have tried to write the code to be as clean and well designed as possible. XAudio2 should be considered a model plugin for interested developers: it uses the SDK in a well structured and effective way. The ACE functions are thin wrappers to the underlying CXAudio2 class, errors are handled by exceptions and passed to the debugger, and serialization of setup parameters is handled by a class inheriting CXAudio2's setup parameters. When the documentation and examples are also finished and it is out of beta, these also are intended to be ideal examples for other developers (and hopefully where all official plugins will end up). Edit: documentation subsequently completed
Hope the code helps potential developers, more on the way soon!