Currently, my work environment has only RHEL 5.4. I'd like to try some other Linux distributions, such as Ubuntu, with virtual machines.
Is there any free virtual machine software applicable to linux?
For Desktop enviromemnt you can use VirtualBox, for the server virtualization i suggest KVM+libvirtd+virt-manager that work very well.
I just read an article in the Dutch Linux Magazine. For Servers, they said that Xen is the best and most mature solution. Xen has been used longer by all main distro's except Ubuntu and they said that's only because Ubuntu never got a stable Xen Hypervisor in their releases (if I remember correctly).
They also said that KVM is less mature and not really used in (large) enterprises.
Our colocator (second largest in The Netherlands) also prefers Xen, but then Citrix Xenserver, which is not open source.
And in my experience; I have one box with KVM on Debian Lenny, and now and then, the virtual machine crashes. And, which is the biggest fail of KVM on Debian, no provision has been made to shutdown the VM's when you shut down the host; they're not shut down and therefore killed when the machine powers down. This is unforgivable to me.
http://www.virtualbox.org/ <- is very good
For some reason I can not comment, so I posted it as an another answer:
Is the future of VirtualBox in jeopardy because of the Sun-Oracle acquisition?
I'm sure that there are more options for specialized needs, but for my general use Virtual Box has been fantastic.
After a summer evaluating a dozen or so platforms, I settled on Proxmox. Basically a minimal Debian-based bare-metal hypervisor with KVM, QEMU & OpenVZ containers.
It uses a non-C++ implementation of libvirt to keep it lightweight. Licensing & stability are non-issues. Performance is very good.
Virtualbox is good for a desktop-based host, for occasional use.