Always excellent when a piece of computing history has it's source made available. Even better when it's provided under a libre license, as this is! CP/M is a

Always excellent when a piece of computing history has it's source made available. Even better when it's provided under a libre license, as this is! CP/M is a powerful example of what you can do without multitasking. The code is also a time capsule of development techniques and styles of the time.