Linux Kernel source tree
Scalable, optimized, secure RTOS for multiple hardware architectures
Market leading real time kernel for 40+ microcontroller architectures
Real-Time Event Frameworks based on active objects & state machines
Cluster wide IPC providing datagram, connection, and bus messaging
Tool kit for building highly secure special-purpose operating systems
freeRTOS for Arduino Uno, Goldilocks 1284p & Mega 2560
Scripts to build a custom Linux system for 16MB Netpliance I-Opener.
Simplest kernel for cooperative multithreading
Free real-time operating system designed for microcontrollers
RTOS for Secure, Safe and Real-Time Systems
Real time preemptive embedded multi-thread kernel
Real-Time Embedded Operating System
Open-source RTOS and Embedded Systems Toolkit for C++ and C
PRTOS is a simple, compact preemptive RTOS for embedded systems
ext2/3/4 library for microcontrollers
W-Bus communication library