On a desktop computer, you think of an operating system as a big piece of complex software. For small systems (like an Arduino) you might want something a lot simpler. Object Oriented State Machine Operating System (OOSMOS) is a single-file and highly portable operating system, and it recently went open source.
OOSMOS has a unique approach because it is threadless, which makes it easy to use in memory constrained systems because there is no stack required for threads that don’t exist. The unit of execution is a C++ object (although you can use C) that contains a state machine.
RapidDisk is an advanced Linux RAM Disk which consists of a collection of modules and an administration tool. Features include: Dynamically allocate RAM as block device. Use them as stand alone disk drives or even map them as caching nodes to slower local disk drives.
I pushed it into the mainline last Sunday. Yes, I know, I am a bit late with this announcement.
The Solus operating system is still under construction, and it looks like a lot of cool packages are getting implemented, including a new one called DoFlicky that will help users to install drivers much easier.