Chapter 3. The xUnit Family of Unit Test Frameworks
Kent Beck published a unit test framework for the Smalltalk language in 1999. The architecture of SmalltalkUnit (or SUnit) represents a sweet spot, an ideal balance between simplicity and utility. Later, Erich Gamma ported SUnit to Java, creating JUnit. JUnit in turn begat CppUnit, NUnit, PyUnit, XMLUnit, and ports to many other languages. A dizzying array of unit test frameworks built on the same model now exists. These frameworks are known as the xUnit family of tools. All are free, open source software.