Virtual iron allows you run any OS within any OS .For ex.:-using Virtual Iron you can run Windows Xp/Vista in any Linux distors or vice versa.It started as software that allowed a single OS to use in multiple server.In contrast VM- ware and MS Virtual PC allow you to run many OS on single server.
This is an answer to VMware and MS Virtual Pc so why not give a try and you wont like switching to earlier Virtual Software 
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[li]Virtual Iron Virtualization Manager Virtual Data Centerprovides a central place to control and automate virtual resources. It streamlines tasks that are normally highly manual and time-intensive and significantly reduce data center costs and complexity.[/li][li]Virtual Iron Virtualization Services are deployed automatically on bare-metal, industry-standard servers without requiring software installation or management. These features streamline data center management and reduce operational costs.[/li][li]Open Source Virtualization, based on an open source hypervisor derived from the open source community project, is the first software loaded when the physical server boots. The hypervisor manages all hardware resources and leverages the hardware-assisted virtualization capabilities built into Intel and AMD processors to create an abstraction layer between physical hardware and virtual resources.[/li][/ul]
Downlaod: Free Download
There is also one more freeware called VirtualBox.
Link: VirtualBox
I have tried VirtualBox and its gr8.
Thanks for ingo on one more freeware for virtualization.
how is the guest OS performance using these tools as compared to VMWare?