µGFX - The GUI library for microcontrollers
µGFX can either be run on top of an operating system like ChibiOS/RT, FreeRTOS and many others or on a bare-metal system.
The library is entirely written in C. It can be used in C++ applications without any modifications.
It's free to use without any usage restrictions for non-commercial products and open hardware projects. Commercial licenses are available at low prices.