NuttX
Real-Time Embedded Operating System
Nuttx is a real time embedded operating system (RTOS). Its goals are: (1) small footprint usable in deeply embedded, resource constrained environments, (2) fully scalable from tiny (8-bit) to moderate (32-bit), (3) standards compliance, (4) real time, and (5) totally open. Think "Tiny Linux".
NuttX is now and Apache project in incubation. See https://nuttx.apache.org/download/ for the latest downloads.