RecoreFX fills the most common needs for C# code after the .NET standard library.
Recore includes a variety of collection and LINQ extensions as well as some best practices that most codebases end up implementing from scratch.
Recore also brings some F#-specific features, such as Optional and Of, to C# and other languages targeting .NET.
Install from NuGet:
dotnet add package RecoreFX
There are other packages offering more LINQ-style methods, better URI handling, or implementations of Optional.
These packages admittedly have richer feature sets than Recore has, and may be right for your project if you want that depth of functionality.
RecoreFX is designed to feel like a natural extension of CoreFX. It's meant to be a one-stop shop, filling in the most common use cases without a steep learning curve.