Top 20 NuGet assertion Packages

Shouldly - Assertion framework for .NET. The way asserting *Should* be
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 simple package with guard clause helper methods. See docs for how to extend using your own extension methods.
HttpClient extensions to test REST endpoints.
A high-performance, extensible argument validation library.
Simple, fluent, lightweight assertion library to create assertion points in your code.
Simple fluent assertions for .NET.
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 ...
Testify is a unit test assertions, test data creation and contract verification framework. It's not dependent on any specific unit testing framework.
Testify is a unit test assertions, test data creation and contract verification framework. It's not dependent on any specific unit testing framework. Testify.Moq adds mocking support via Moq.
Assert Extensions for the Microsoft Test Framework - MSTest V2
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 collection of xUnit.net assertions, tailored for Tiger libraries. (As source.)
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
A minimalistic guard library with Resharper contract annotations
Test Run Extensions for the Microsoft Test Framework - MSTest V2