Red Hat Enterprise Virtualisation is based on the RH-incubated oVirt project - an open-source management layer around multi-host KVM. In my experience though it had many rough edges and is very opinionated on how you setup your cluster, not reacting well to mixtures of hardware types and storage back-ends.
Also there's the re-incarnation of XenServer... XCP-ng with Xen Orchestra. Never used it, looks promising. I was a fan of proper original Xen many years ago.
I have not looked at what's happening in the OpenStack world for some time. Last I looked it was mostly bare-bones platform-as-a-service type projects rather than a fully integrated virtualisation platform.
Proxmox is worth looking at for smaller clusters and is more polished and flexible than oVirt/RHEV. If I was looking for a budget homelab or other cheap setup I would go with it.
Thanks for the tip on OpenNebula ^ I'd never heard of it and will have to check it out.
Honestly though when you compare the flexibility of cluster hardware support, mixture of storage types, live storage and compute migration... and vSAN... you have to be mad to look at anything other than VMware vSphere for small biz and larger org non-cloud/on-prem. I don't even particularly like vSphere but for the most part it "just works™" moreso than any competitors. I have been paying for VMUG EvalExpress license for some years for my homelab tinkering.