NUnit
.NET test framework. Port of Java JUnit to .NET (Jim Newkirk + Charlie Poole), 2002. The first major unit testing framework on the .NET platform.
v3 (2015) total redesign. Attribute-based: [Test], [TestCase], [SetUp]. Competitors: xUnit.net (Jim Newkirk pivoted there in 2007, more of a Microsoft favorite), MSTest (Microsoft's official, fewer features). Modern .NET projects often pick xUnit for defaults + Microsoft backing, but NUnit has more features (parameterized tests, parallelism, theory-style data).