eCos is an open source, configurable, portable, and royalty-free embedded Real Time Operating System. It is written in C++, provides all the usual OS things. You configure the OS so that the parts you don't need aren't loaded to the embedded system.
Cos (embedded Configurable operating system) is an open source, royalty-free, real-time operating system intended for embedded systems and applications which need only one process with multiple threads. The OS is configurable, and can be customised to precise application requirements, with hundreds of options, delivering the best possible run-time performance and minimized hardware needs. A thriving net community has grown up around eCos ensuring ongoing technical innovation and wide platform support.
Cos is also the name of a real-time operating system.