A new version of an Arduino Realtime OS is on the go:

https://github.com/greiman/ChNil

I have used with profit and fun NilRTOS, and I strongly suggest to you this new variant.

A preemptive multi tasking RTOS in a 2KB 1-MIPS, 8 bit microcontroller remember be the ugly time when a MS-Windows 3.x with 1 MB of RAM was an inferior operative system no-preemptive at all, but with nice software like Excel and Access