CLI

• Typin - Simple to use declarative framework for interactive CLI applications and command line tools (direct mode) that has its roots in CliFx.
• EntryPoint - Composable CLI Argument Parser for .NET Core & .NET Framework 4.5+.

ETL

• Cinchoo ETL - ETL Framework for .NET (Read / Write CSV, Flat, Xml, JSON, Key-Value formatted files)

HTML and CSS

GUI - other

• AngleSharp - Complete HTML5 DOM and CSS3 OM construction

Interoperability

GUI - other

• CppSharp - Tools to surface C++ APIs to C#

JavaScript Engines

GUI - other

• 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.

Mail

GUI - other

• MimeKit - A cross-platform .NET MIME creation and parser library with support for S/MIME, PGP, TNEF and Unix mbox spools.

Misc

GUI - other

• RecordParser - Library to help reading and writing CSV and Flat files with zero heap allocation.
• YoutubeExplode - Ultimate library for extracting metadata and downloading Youtube videos and playlists.

Office

GUI - other

• ExcelDataReader - Lightweight and fast library written in C# for reading Microsoft Excel files (2.0-2007).

Parser Library

GUI - other

• Silverfly - A pratt parser library.
• Pidgin - A lightweight, fast and flexible parsing library for C#, developed at Stack Overflow

Template Engine

GUI - other

• Fluid - Fluid is an open-source .NET template engine based on the Liquid template language.