Contiki is an open source, low-power operating system for networked embedded systems. It includes the worlds smallest IPv6 stack and provides full IP networking in a few kilobytes of RAM, as well as low-power mechanism that makes a tiny radio-equipped microcontroller run for years on a pair of batteries. Version 2.4 has been released today.
As you would expect, this new release brings improvements (no way!), a number of bugfixes, and improved stability.
The low-power wireless MAC protocols have gotten an overhaul, improving power-efficiency and improved collision and interference handling. The COOJA/MSPsim simulation environment has received a significant speedup. Two new experimental platforms are included: the Crossbow MicaZ and the Sensinode CC2430/8051 platform. Many improvements and bugfixes has been made to the uIP code as well as the SICSlowpan implementaion of 6lowpan IPv6-over-802.15.4.