Virtualization! What does it mean? If you are not aware of this concept then article at http://www.kernelthread.com/publications/virtualization/ is good read.
However, below is the list of virtualization providers that I can think of.
+ VMware – VMware Player and VMware Server are one of the free products that are available. However, it requires registration. Other virtualization products are also available.
+ Parallels Virtuozzo - Creates isolated virtual environments (VE) or containers on a single physical server and OS instance.
+ KVM – An Open Source full virtualization solution for Linux on x86 hardware
+ Sun VirtualBox – An Open Source virtualization product.
+ Solaris Zones, LDOMs
+ OpenVZ – A container-based virtualization for Linux.
+ Linux-VServer – A virtualization for GNU/Linux systems
+ QEMU – A generic and open source machine emulator and virtualizer.
+ UML – User Mode Linux
Are you aware of any other virtualization provider? Then do let everyone know in comments below.