User Mode Linux is another mean to run independent, safe and secure Virtual Linux distributions, versions and processes on your main Linux system.
Unfortunately, often unknown by System Administrators.
Ok, it use paravirtualized technique but if you are using Linux Virtual machines on Linux Physical Machine it is most of the time responding to your basic needs – e.g. web servers, and so on -.
You will be able to use it on Fedora 10 with the following package : uml_utilities.i386 0:20070815-5.fc10
Otherwise you can download it on the UML website
It contains uml_net, uml_mconsole, uml_switch, tunctl, and a number of other useful utilities in order to use the user-mode linux kernel.
You will find a great article at the following URL : http://www.redhat.com/magazine/001nov04/features/usermode/
Obviously, It exists also other techniques, methods in order to run VM – virtual machines – on Linux and VmWare is probably one of the most known.- Marketing have done its job, you see what I mean 
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However, it is not always the right answer to your needs and requirements.
Here is a list of Open source Virtualized project as well as their associated types and licences:
Emulation type :
Bochs – Licence -> LGPL -
QEMU – Licence -> LGPL/GPL -
Paravirtualization type :
UML - Licence -> GPL -
Paravirtualization and Full virtualization type – with v-processors (Intel, Amd) – :
Xen - Licence -> GPL – (own by Citrix now)
Operating system-level virtualization type :
Linux-VServer - Licence -> GPL -
OpenVZ - Licence -> GPL -
Full Virtualization
KVM – Licence -> GPL (supported by Red Hat)
Virtualbox – Licence -> GPL
Proprietary Solution & Full Virtualization :
VmWare – Licence -> Proprietary Solution –
Now, it is up to you to choose the application which best fit your strategy, your requirements, your budgets and your needs.
BTW, Thanks to Eddy to have shown me what I did not see.
