eamon 

Generates valid, readable C# from an expression tree, and can annotate that code with runtime values. Useful for e.g. code generation and unit testing assertions.

Collection of utilities developed by ProgressOnderwijs

Efficient hash code and equality implementations for arbitrary objects.

Find the english language indeterminate article ('a' or 'an') for a word. Based on real usage patterns extracted from the wikipedia text dump; can therefore even deal with tricky edge cases such as acronyms (FIAT vs. FAA, NASA vs. NSA) and odd symbols. (Requires .NET Core 1.0 or .NET 4.5)

Zlib (deflate) compression including dictionary support. This is a wrapper around DotNetZip.

Incrementally computes the variance, mean and count without needed several passes.

Sorts Span<T> (and arrays) more quickly than System.Array.Sort