Nanos is a "unikernel" providing a subset of Linux kernel features. It only allows a single process to run, but has multiple threads. It maintains the kernel-land to user-land boundary unlike other unikernels (so perhaps isn't strictly a unikernel).
I want to use it to run apps in VMs. It's very good for this because it behaves like a container from scratch, only pulling in the files it needs. The kernel itself is very small and lightweight. It's possible to produce very small VMs that boot very quickly. More so perhaps than Linux containers running in firecracker microvms.