Algorithms and Data structures

• Towel - data structures, algorithms, mathematics, metadata, extensions, console, measurements, and other useful stuff

Assembly Manipulation

• Mono.Cecil - Cecil is a library to generate and inspect programs and libraries in the ECMA CIL form.

CLI

• Typin - Simple to use declarative framework for interactive CLI applications and command line tools (direct mode) that has its roots in CliFx.
• CsConsoleFormat - .NET C# library for advanced formatting of console output.

Database

• ZoneTree - Persistent, high-performance, transactional, and ACID-compliant ordered key-value database for .NET.

GIS

• NetTopologySuite
• DEM Net Elevation API - .Net library for Digital Elevation Models, allows 3D terrain generation in glTF / STL format.

HTML and CSS

GUI - other

• AngleSharp - Complete HTML5 DOM and CSS3 OM construction

JavaScript Engines

GUI - other

• Jurassic - A implementation of the ECMAScript language and runtime. It aims to provide the best performing and most standards-compliant implementation of JavaScript for .NET.
• YantraJS - JavaScript Runtime (Similar to NodeJS) for .NET Standard, compiles JavaScript to IL, support for many ES6 features, generators, CommonJS modules, CSX modules and expression compiler.

Misc

GUI - other

• ComputeSharp - A a .NET library to run C# code in parallel on the GPU through DX12, D2D1, and dynamically generated HLSL compute and pixel shaders.

PDF

GUI - other

• IText - iText is a PDF library that allows you to CREATE, ADAPT, INSPECT and MAINTAIN documents in the Portable Document Format (PDF)[$] [Free for OSS]