raygui
A simple and easy-to-use immediate-mode gui library
...Because the library is distributed as a single header file containing both declarations and implementations, including it in a project is simple and doesn’t require complex build configurations. raygui supports a wide range of controls and customization options for colors, fonts, and layout metrics, and also includes a built-in set of 1-bit icons that can be embedded into controls. It’s intended for tools development, rapid GUIs for editors or debug interfaces, and simple applications that need basic UI without heavy frameworks.