ReSharper's functionality can be significantly extended by means of its open API. In fact, most of ReSharper's features are implemented using this same API that is available in ReSharper plugins.
It is possible to add the following functionality to ReSharper:
- Highlighting of new errors and warnings (C# only)
- New quick-fixes (C# only)
- Custom tool windows
- New refactorings
- And more
For more information on using ReSharper's Open API, see http://www.jetbrains.net/confluence/display/ReSharper/ReSharper+Plugin+Development.
You can also glance through the list of available ReSharper plugins.