pico]OS is a highly configurable and very fast real time operating system (RTOS). It targets a wide range of architectures, from very small 8 bit processors and microcontrollers up to very huge platforms. Ports are available for ARM, AVR, PPC, MSP430, 6502, MYCPU and for developing and testing purpose also for 8086, Win32 and Unix.
pico]OS 1.0.4 was released 2012-07-12. This is a maintenance release that includes some bug fixes for the new ports for MSP430 and ARM Cortex-M.
You may also visit Ari's website that contains cool stuff related to pico]OS: http://stonepile.fi/tags/picoos/