NUnit is designed to be extended in a number of ways.
Extensions to the NUnit framework - the part of NUnit that is referenced by tests - usually take the form of Custom Constraints, written by users to encapsulate tests that pertain to their specific projects.
Extending the features found within NUnit itself depends on the use of NUnit Addins. Currently, The Addin mechanism only supports extensions to the NUnit core - the part of NUnit that builds and executes test suites. However, the API that is used provides for the future ability to extend the client side of NUnit, including the GUI.