We have some virtual machines on vmware server 2.x running on 64 bit hardware and quite happy with it.
As vmware server will no longer be offered we are thinking to migrate to ESXi, which seems is free. We will have to install the specialized network cards but that's a minor problem.
But once left alone with a quite silently discontinued product there is some resistance to vmware.
VirtualBox seems to work: http://blogs.oracle.com/virtualization/2010/06/migrating_from_vmware_to_virtu.html
What other free (of licencing cost) options are there? We have windows server 2003 32 bit VMs and also linux 32 and 64 bit VMs to migrate. So xen does not seem an option, which does not run microsoft OSes.
Edit: I accepted grub's answer, because I find many aspects of the questions covered. But its not easy to choose, because all answers are very good. Thank you all! I feel quite well informed now and your answers really helped a lot.
My suggestion would be to go with ESX/ESXi -- You presumably already have a substantial investment in VMWare knowledge in-house, and those skills mostly transfer over to managing ESX/ESXi. In addition if you ever want to leverage the advanced features of the VirtualCenter platform an already-installed ESX environment is a great starting point.
The other thing to consider is that if you have problems it's really easy to find VMWare people (at least here in NY, and in my experience on the internet too) -- Other virtualization solutions are great but you may have to do a little digging to find help if something goes wrong.
You can also take a look at the XenServer from Citrix. It's like the ESXi freely available. I don't have much experience with the XenServer and don't now it's unlicensed feature set.
I would recommend ESXi because the migration of the VM's from the VMware Server to ESXi should be nearly painless (thanks to VMware Converter). Also you already got the basic knowledge about the VMware envionments (naming conventions etc.)
VMware will not discontinue ESXi 4 or change it's pricing (basicly free...). At least not in the near future.
The end of life of the Vmware server product line wasn't "silently". You just have to know where to look ;-)
http://www.vmware.com/support/policies/lifecycle/general/index.html#policy_server
ESXi is limited in certain capabilities but free. Virtualbox is more purposed for workstations...if you need to virtualize something to test or run for a specific purpose, rather than using it to run a 24/7 server.
Xen on certain hardware can run Windows. See the Wiki article for more information to start from.
Hyper-V should also have a free version available, as I recall, which is basically like their competitive answer to ESXi. I haven't used it though.
What is the problem with just using ESXi? You said there's resistance but is there a particular reason that's valid or is it just political? If you're running servers the best options would be probably hyper-V or ESXi, but it depends on what you're using the virtualized machines for and what you're familiar with. Seems kind of silly if you're familiar with VMWare to just toss it away if you don't have valid reasons to move away from it.
If your CPU has virtualization support you can run Windows based VM's on Xen just fine. Your solution of choice may depend on your current expertise (windows or linux centric), the infrastructure you have in place (hypervisor <-> Hardware compatibility), and your long term virtualization and growth roadmap. Pick the right product now that fits your long term plans to avoid (painful & lengthy) future V2V conversions.
I migrated my workplace from VMware Server to XenServer and the results have been excellent. Live migration and central management without a $25,000 pricetag (that's what VMware quoted me for VSphere). Been in operation for six months without a hiccup.