Updated C++ source code for my GIFAnim, NFO and TextThumb plugins is now available from my web page:
http://www.pretentiousname.com/pnop_source/index.html
Source for the JP2Raw and Ogg plugins is also there (but is not new).
On the page you will see that I've written a little bit about each plugin from a programmer's perspective. Hopefully this makes the examples more useful as you can get an idea of the different kinds of plugins that can be written and see which plugins would serve as good examples for your particular project. (This certainly isn't an exhaustive list, though. There are no VFS (virtual file-system) plugins at all yet, for example.)