Here's one that is "production" ready: the Mirage-Firewall microkernel built using MirageOS and running on Qubes OS.[0] In general MirageOS allows you to:

> ... construct unikernels for secure, high-performance network applications across a variety of cloud computing and mobile platforms. Code can be developed on a normal OS such as Linux or macOS, and then compiled into a fully-standalone, specialised unikernel that runs under a Xen or KVM hypervisor.[1]

[0] : https://github.com/mirage/qubes-mirage-firewall

[1] : https://mirageos.org/