GoogleTest is Google's C++ mocking and test framework. It's used by many internal projects at Google, as well as a number of notable projects such as The Chromium projects, the OpenCV computer vision library, and the LLVM compiler.
This GoogleTest project is actually a union of what used to be two separate projects: the old GoogleTest and GoogleMock, an extension of GoogleTest for writing and using C++ mock classes. Since they were so closely related, they were merged to create an even better GoogleTest.
GoogleTest features an xUnit test framework, a rich set of assertions, user-defined assertions, death tests, among many others. It's been used on a variety of platforms, including Cygwin, Symbian, MinGW and PlatformIO.