Text copied from the official Contiki website.
What is Contiki?
The Contiki operating system is a highly portable, minimalistic operating system for a variety of constrained systems ranging from modern 8-bit microcontrollers for embedded systems to old 8-bit homecomputers. Contiki provides a simple event driven kernel with optional preemptive multithreading, interprocess communication using message passing signals, a dynamic process structure and support for loading and unloading programs, native TCP/IP support using the uIP TCP/IP stack, and a graphical subsystem with either direct graphic support for directly connected terminals or networked virtual display with VNC or Telnet.