FreeRTOS is a popular choice among many as it is free, supported and has its own ecosystem of middleware.

It is has support for various architecture including M4K (PIC32) which is officially supported by FreeRTOS and MIPS32 which is the contributed port provided by Imagination Technologies for all modern MIPS32 cores covering mip32r2 to mips32r6 architectures.

To get started on MIPS32 please visit our contribution page

http://interactive.freertos.org/forums/21172749-Imagination-Technologies-MIPS

General information about FreeRTOS API, middleware and source code can be found in their official page at www.freertos.org