NUnit is described as 'unit-testing framework for all .Net languages. Initially ported from JUnit, the current production release, version 2.5, is the sixth major release of this xUnit based unit testing tool for Microsoft .NET'. There are three alternatives to NUnit for Windows, Linux and Mac. The best alternative is JUnit, which is both free and Open Source. Other great apps like NUnit are XUnit (Free, Open Source) and TestNG (Free).
JUnit is a unit testing framework for the Java programming language. JUnit has been important in the development of test-driven development, and is one of a family of unit testing frameworks collectively known as xUnit that originated with SUnit.
xUnit.net is a developer testing framework, built to support Test Driven Development, with a design goal of extreme simplicity and alignment with framework features. It is compatible with .NET Framework 2.