Contiki

"Contiki isn’t nearly so well-known as Windows or OS X or even Linux, but for more than a decade, it has been the go-to operating system for hackers, academics, and companies building network-connected devices like sensors, trackers, and web-based automation systems. Developers love it because it’s lightweight, it’s free, and it’s mature. It provides a foundation for developers and entrepreneurs eager to bring us all the internet-connected gadgets the internet of things promises, without having to develop the underlying operating system those gadgets will need.

Perhaps the biggest thing Contiki has going for it is that it’s small. Really small. While Linux requires one megabyte of RAM, Contiki needs just a few kilobytes to run. Its inventor, Adam Dunkels, has managed to fit an entire operating system, including a graphical user interface, networking software, and a web browser into less than 30 kilobytes of space. That makes it much easier to run on small, low powered chips–exactly the sort of things used for connected devices–but it’s also been ported to many older systems like the Apple IIe and the Commodore 64."  http://www.wired.com/2014/06/contiki/