The seL4 kernel is aimed to be used by highly secure and reliable systems. The open-source seL4 project proclaims, "the world's first operating-system kernel with an end-to-end proof of implementation correctness and security enforcement. It is still the world's most highly-assured OS." The seL4 developers also believe, "to the best of our knowledge, seL4 is the world's fastest microkernel on the supported processors, in terms of the usual ping-pong metric: the cost of a cross-address-space message-passing (IPC) operation."
The seL4 kernel source code is opened up under the GPLv2 and 2-clause BSD license. The timing of this open-sourcing was done in honor of today, The International Proof Day, with this being five years since the completion of seL4's functional correctness proof.
SeL4 supports ARMv6, ARMv7, and x86 hardware. SeL4 is capable of running on Intel Haswell x86 hardware and older while the supported ARM platforms range from the OMAP3 BeagleBoard to the Exynos ODROIDs on the ARM side. It is worth noting that seL4 is able to run Linux on top in a virtual machine on x86. You can learn more via the seL4 FAQs.
Those wishing to learn more can visit seL4.systems. The code to the seL4 kernel is hosted on GitHub.