Terminal.Gui v2
Terminal.Gui is a cross-platform .NET toolkit for building sophisticated terminal UI apps on Windows, macOS, and Linux.

The original author, Miguel de Icaza, graciously allowed me to take it over and maintain it. It is now the most popular library for building TUI apps in .NET, with menus, forms, tables, trees, dialogs, text editing, themes, key bindings, mouse support, TrueColor, Unicode, and inline or full-screen app models.
Terminal.Gui v2 is the current stable line. It introduced a cleaner application architecture, stronger layout and view systems, better configuration, modern docs, templates, and a migration path for v1 users. The project now lives under the tui-cs organization. Verified current release: v2.4.17.