Google's open-source implementation of the Thread networking stack (IEEE 802.15.4, 6LoWPAN, MLE, commissioning). Flashed to a supported radio (e.g. nRF52840) it becomes an NCP/RCP you drive over Spinel, and with pyspinel that same device turns into an 802.15.4 packet sniffer or a node that can scan for and try to join a Thread mesh.
Lifecycle not assessed: nobody has checked yet whether this tool is still maintained, sold or working. Check the project's own page before relying on it.