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.
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 high-performance, extensible argument validation library.
Generalized assertion library, covering argument, operation, and unit test assertions.
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 ...
Simple fluent assertions for .NET.
A set of assertions for FluentAssertions which provides strong-typed assertions for instances of Optional.
Testify is a unit test assertions, test data creation and contract verification framework. It's not dependent on any specific unit testing framework.
Assert Extensions for the Microsoft Test Framework - MSTest V2
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.)
A fluent assertions library that performs full coverage on enumerable types.
Extensions for VisualStudio UnitTest's Assert class to simplify e.g. testing of view models.
Diagnostic functionality similar to Debug.Assert but with more features. Assertions can be exercised by unit tests without the debug dialog getting in the way. Full documentation is included in the NuGet package.
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
Simple, fluent, lightweight assertion library to create assertion points in your code.
HttpClient extensions to test REST endpoints.