Dear ImGui is an extremely popular open source C++ GUI immediate mode framework used for many games and game engine/tool applications. ImGui has been used to create hundreds of games, engines, tools and applications, including several AAA games. Users include large game studios such as Blizzard, Ubisoft and Activision as well as corporations such as NVidia and Google. There are language bindings for dozens of programming languages and game engines as well as several rendering back-ends.
ImGui describes itself as:
Dear ImGui is a bloat-free graphical user interface library for C++. It outputs optimized vertex buffers that you can render anytime in your 3D-pipeline enabled application. It is fast, portable, renderer agnostic and self-contained (no external dependencies).
Dear ImGui is designed to enable fast iterations and to empower programmers to create content creation tools and visualization / debug tools (as opposed to UI for the average end-user). It favors simplicity and productivity toward this goal, and lacks certain features normally found in more high-level libraries.
Dear ImGui is particularly suited to integration in games engine (for tooling), real-time 3D applications, fullscreen applications, embedded applications, or any applications on consoles platforms where operating system features are non-standard.
Dear ImGui 1.80 was just released with tables being the primary new feature now in master, alongside dozens of other changes and fixes. You can learn more about Dear ImGui in the video below. Dear ImGui is open source under the MIT license with the source code available on GitHub.