NitrOS-9 is a real-time, process-based, multitasking, multi-user, Unix-like operating system for Tandy CoCo, Dragon, and several other Motorola MC6809-based computers. The original OS-9 was created in 1979. NitrOS-9 is the modern equivalent of that OS, and includes advanced features like support for up to 2 MB RAM and 4 GB Hard drive partitions. It is still being developed, and support is available in many mailing lists and forums.
Please see the official NitrOS-9 wiki for documentation.