Crosos is a SubLinux Distribution
Crosos is a open-source OS. It is based on the Linux Kernel and mostly uses the same code in Ubuntu (but it has been derived from Debian)
FreeMiNT is a free alternative OS kernel for Atari systems & clones
FreeMiNT provides the basis of a free TOS compatible replacement OS that can multitask. Used together with the free system components fVDI (device drivers), XaAES (graphical user interface widgets), and TeraDesk (desktop manager), it provides a modern user interface.
MightyOS is a serious attempt at making a fast and lightweight Operating System written in C++.