Nuttx is a real timed embedded operating system (RTOS). Its goals are: (1) small footprint usable in deeply embedded environments, (2) fully scalable from tiny (8-bit) to moderate (32-bit), (3) standards compliance, (4) real time, and (5) totally open.
http://www.linuxlinks.com/portal/news/article.php?story=20070401161138313