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.