While there have been cut-down Linux implementations before the increase in numbers of smart, connected devices has made some thing a little more specialised more important.

Zephyr is all about minimising the power, space, and cost budgets of IoT hardware.
For example a cut down Linux needs 200KB of RAM and 1MB of flash, IoT end points, which will often be controlled by tiny microcontrollers.

Zephyr has a small footpoint “microkernel” and an even tinier “nanokernel.” All this enables it to be CPU architecture independent, run on as little as 10KB while being scalable.

It can still support a broad range of wireless and wired technologies and of course is entirely open saucy released under the Apache v2.0 License. It works on Bluetooth, Bluetooth Low Energy, and IEEE 802.15.4 (6LoWPAN) at the moment and supports supports x86, ARM, and ARC architectures.