* Xen (https://help.ubuntu.com/community/Xen) is the Ubuntu-supported virtualization platform used for cloud computing. it is the basis for the Amazon EC2 cloud.
* Another Ubuntu-supported virtualization system is called KVM (https://help.ubuntu.com/community/KVM). It is officially supported, but is somewhat harder to set up for the average user. It also requires hardware virtualisation extensions in your CPU.
* VMware (https://help.ubuntu.com/community/VMware) is a proprietary alternative, with a free virtual machine player available. A free server for Linux can also be found.
* QEMU (https://help.ubuntu.com/community/Qemu) is the original virtualization system, on which Virtualbox and KVM are based. It is completely open source and can be used with or without the virtualization hooks found in many new processors.
It is possible to convert virtual machines between these systems (but it is sometimes a time consuming process). A new "open virtualization format" has been created that can be used in several alternative virtual machines.