I don't think there is another notable, proven, open source clustered manager of both actual VMs and containers. OpenNebula might be the closest: https://opennebula.io/get-your-hands-on-v-5-8-edge/
LXD is similar to proxmox if you only need containers, and pretty slick. They use their own distributed version of sqlite to propagate state, so it's much simpler than proxmox.
Googles Ganeti is strikingly similar to Proxmox, to the point where I suspect Proxmox started as a reimplementation:
http://www.ganeti.org/ (developed at https://github.com/ganeti/ganeti)
It does not come with a GUI, though there are a few third party ones. It has API endpoints for everything, a fully featured CLI client, and is extremely stable.
Googles marketing department does not want you to know this, but Ganeti runs most of its internal infrastructure (not public facing stuff).
No affiliation, just a happy user.
Thanks for checking the dates, I thought Proxmox was newer.
The basis of Proxmox VE is the real time replicated clustered configuration file system: https://pve.proxmox.com/pve-docs/chapter-pmxcfs.html
I do not see anything similar for ganeti, also not much similar in general besides that it manages VMs?
Indeed the similarity with Ganeti is only on the surface: being a scalable infrastructure management tool, DRBD as a first-class supported disk backend, etc.
The cluster replication and consensus model is very different: Ganeti has a single "master" node that makes all the decisions and replicates cluster state to other nodes.