Top 20 NuGet assertion Packages

Shouldly - Assertion framework for .NET. The way asserting *Should* be
A simple package with guard clause helper methods. See docs for how to extend using your own extension methods.
A high-performance, extensible argument validation library.
CodeJam is a set of handy reusable .NET components that can simplify your daily work and save your time when you copy and paste your favorite helper methods and classes from one project to another.
Generalized assertion library, covering argument, operation, and unit test assertions.
Simple fluent assertions for .NET.
Emitted or reflected operations on collections of value properties
HttpClient extensions to test REST endpoints.
Assert Extensions for the Microsoft Test Framework - MSTest V2
A fluent assertions library that performs full coverage on enumerable types.
A JavaScript library with function argument assertions, a function parser and assertion wrapper functionality. By wrapping your functions you get defined, notNull, bool, func, number, object, string and notEmpty assertions for free. The interface is specially optimised for the Visual Studio JavaScri...
A collection of xUnit.net assertions, tailored for Tiger libraries.
A minimalistic guard library with Resharper contract annotations
Testify is a unit test assertions, test data creation and contract verification framework. It's not dependent on any specific unit testing framework.
Assertion Error (assertion-error) binding library for Bridge.NET projects.
A collection of xUnit.net assertions, tailored for Tiger libraries. (As source.)
Extensions for VisualStudio UnitTest's Assert class to simplify e.g. testing of view models.
Use code snippet 'assertion' to get the declaration at the top of the file. Then just use Assert.Equal(...) and the like.
Guardly is a defensive programming framework
Little helper on assertion.