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.
Generalized assertion library, covering argument, operation, and unit test assertions.
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.
A set of assertions for FluentAssertions which provides strong-typed assertions for instances of Optional.
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 ...
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.
Assert Extensions for the Microsoft Test Framework - MSTest V2
Little helper on assertion.
A collection of assertions, tailored for Tiger libraries.
A minimalistic guard library with Resharper contract annotations
Assertion Error (assertion-error) binding library for Bridge.NET projects.
A collection of 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.
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