Arduous is a preemptive Arduino kernel for the masses. It is being actively developed by @snemanden and @roevhat over the summer. It aims to feature both round-robin and fixed priorities as scheduling policies. Also, semaphores and message queues are on the to-do list. The goals is to create a light and understandable kernel, with simple system calls and a minimum of overhead for the user/application.
This kernel could not be without the help of @jdn-aau, who wrote the SNOT kernel for Arduino. If you are in need of a functional preemptive kernel right now, SNOT is worth looking at!