eCos (embedded configurable operating system) is a free and open source real-time operating system intended for embedded systems and applications which need only one process with multiple threads. It is designed to be customizable to precise application requirements of run-time performance and hardware needs. It is implemented in C/C++ and has compatibility layers and APIs for POSIX and µITRON.
Read more about ECos: Design, History, Non-free Versions, Criticisms