Thanks, Riemer, for your appreciation 
It isn't a real OS, I know... But more sophisticated than other schedulers or taskmanagers that use millis(). And it's simpler than an [RT]OS that store and restore the task's status into the stack. I chose leOS because it's a pun with my nick, leo 
And maybe there could be some improvements, who knows about that?
BTW, here is version 0.1.0. It's fully compatible with Arduino Leonardo and its Atmega32U4. Moreover, leOS 0.1.0 supports one-time tasks (a task that should be executed only 1 time and then removed from the scheduler) and the ability to modify the interval and the type of the task. Info on the README file, and more examples in the package.
leOS-0.1.0.zip (24.2 KB)