TestNG is described as 'testing framework inspired from JUnit and NUnit but introducing some new functionalities that make it more powerful and easier to use' and is a Unit Testing Tool in the Development category. There are three alternatives to TestNG for Windows, Linux and Mac. The best alternative is NUnit, which is both free and Open Source. Other great apps like TestNG are JUnit (Free, Open Source) and XUnit (Free, Open Source).
NUnit is a 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.
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.