Looking at the big picture, there's more to ReSharper than its own extensive set of features. ReSharper API offers a rich platform for developing intelligent plugins. These plugins provide support for additional unit testing frameworks, implement special-purpose refactorings, validate naming conventions, and perform many more tasks which any .NET developer will find useful.
How to Install ReSharper Plugins
Since version 8.0, ReSharper contains Extension Manager (ReSharper | Extension Manager in Visual Studio) that helps discover and get additional information on plugins, install and uninstall them.
Users of ReSharper 8.0 and above working with Visual Studio 2010, 2012, 2013 or 2015 should use the Extension Manager to select and install plugins.
If you're using ReSharper 7 or earlier, or if you work in Visual Studio 2005 or 2008, you should find, install and uninstall plugins manually. Please go to the old ReSharper plugin directory and choose applicable plugins from there.
How to Create ReSharper Plugins
If you are developing a ReSharper plugin or if you want to get started, download ReSharper SDK and take a look at ReSharper plugin development documentation. Whenever your plugin is ready, upload it to ReSharper Gallery to make it available to all users of ReSharper versions that you support.