μString is a microkernel or exokernel (call it whatever like you want) - a part of the Jari Operating System Research Project.
For the better support and project life it's licensed under GNU GPLv2 License and you can freely use it in your targets, modify and run this microkernel. μString designed to be stable exists to both source code and API, it's supposed to be a source code backward compatibility that makes a development more effictive compairing with linux kernel for example.
These details are provided for information only. No information here is legal advice and should not be used as such.