Along with Visual Studio templates, the ReSharper SDK installs sample plugins. They are available at C:\Program Files (x86)\JetBrains\ReSharper\v8.1\SDK (if you have 64-bit Windows) and C:\Program Files\JetBrains\ReSharper\v8.1\SDK (if you have 32-bit Windows).
In SDK you can find the following three plugins:
Sample plugin: This presents some basic concepts related to creating custom actions, context actions, background analysis, and extending options.
ReSharper PowerToys: This is divided into a couple of projects and presents more advanced features. Notice that there is no one solution that contains all these projects.
Psi plugin: This presents basic concepts related with developing support for new languages in ReSharper.
You can find more information about these projects in the ReadMe.html file, which is placed together with samples.