For a great number of years, I have been using VMware (vsphere) as the virtual environment for my company’s virtual hosting environment. This has been overall a great product and still is. But a number of years ago I started looking for alternatives because of the high cost of using and keeping vsphere up to date, the licensing cost is quite high. At the time I came across a new product called proxmox that has some fairly good potential to be the replacement. After several months of testing, I came to the conclusion that proxmox just was a little too buggy and was not ready for primetime. Most of this was due to there not being a good backup system in place to create backups. But there were critical bugs at the time that I could just not pass. Despite these issues, I still felt that proxmox had potential and kept an eye on it.
Moving along three or four years and there is now a reasonable backup solution for proxmox and a lot of the original bugs I had have now been resolved so proxmox is now ready to be used in production.
Proxmox is an open-source virtual hosting environment and although proxmox is open source it is a good idea to purchase a support package if you are using it in production so that each machine gets access to production (stable) updates and support for the proxmox environment. This is going to be way less than what you would pay for vmware.