Abassi multicore is a true symmetric multi-processing (SMP) implementation available in two versions, mAbassi and μAbassi. mAbassi is the full feature RTOS and the later is an introductory strip-down version. Every feature of the single core Abassi, using the same intuitive API, is supported in mAbassi, requiring only around 6 Kbytes of code with all features are enable (details). μAbassi is much smaller.
They are available as freeware with some feature limitations (Download).
A feature highlight comparison table between mAbassi and μAbassi is available here.