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.
Generalized assertion library, covering argument, operation, and unit test assertions.
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.
A fluent assertions library that performs full coverage on enumerable types.
Simple fluent assertions for .NET.
HttpClient extensions to test REST endpoints.
Testify is a unit test assertions, test data creation and contract verification framework. It's not dependent on any specific unit testing framework.
Emitted or reflected operations on collections of value properties
A minimalistic guard library with Resharper contract annotations
Assert Extensions for the Microsoft Test Framework - MSTest V2
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...
Easy-to-use fluent assertions with actually useful failure messages.
Little helper on assertion.
A collection of xUnit.net assertions, tailored for Tiger libraries.
Assertion Error (assertion-error) binding library for Bridge.NET projects.
A simple diff comparison asserter that makes it easy to assert if two texts (strings) are equal, and display the differences in a diff tool if they are not. Simply put it is a much simpler (and dumber) version of ApprovalTests that should get the job done for simple scenarios. You can easily create ...
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.