initial versions of the DC++ plugin SDK for C++ and C can be found at https://launchpad.net/dcpp-plugin-sdk-cpp and https://launchpad.net/dcpp-plugin-sdk-c respectively. only the C++ one has received enough love to be useful so far.
the pages there have instructions on how one can create a branch of the plugin (best way to stay up-to-date with the SDK); a direct download is also available.
at the moment, the SDKs contain project files for Visual Studio and a generic makefile (which can then be imported in Eclipse etc). they build a DLL file, which is currently the only way one can load a plugin into DC++. in the future, better, more “automatic” formats are planned; so you will want to keep your plugin branch synchronized.
for examples, https://code.launchpad.net/dcpp-plugin-sdk-cpp lists currently known plugins, some of which make use of boost or lua (so they are not all so trivial).
note also that despite the name of the branch, the plugin SDK is designed to be host agnostic. right now, only DC++ implements it, but chances are other clients may have it as well later on.
this has just started and will probably evolve in the future, so suggestions are very welcome.