The Zephyr OS is a small-footprint kernel designed for use on resource-constrained and embedded systems: from simple embedded environmental sensors and LED wearables to sophisticated embedded controllers, smart watches, and IoT wireless applications.
See the zephyrproject.org site for more information about the project and becoming a member, and this summary of Zephyr resources to help you find your way around the project.
The Zephyr kernel supports multiple architectures, including ARM Cortex-M, Intel x86, ARC, NIOS II, Tensilica Xtensa, and RISC-V 32. For details, see the latest supported boards documentation.
Zephyr is permissively licensed using the Apache 2.0 license (as found in the project's GitHub LICENSE file. There are some imported or reused components of the Zephyr project that use other licensing, as described in Licensing of Zephyr Project components.