I've seen a lot of people who will automatically equate 'virtualisation' with VMWare's suite of software To be fair, most people who have seen virtualisation on their desktops know VMware just because it's the most well known system, and it works on their desktop systems, but it's far from the only option.
There are several different types of virtualisation, but broadly speaking the interesting split is between full machine virtualisation (where you install a complete copy of the operating system into each virtual machine, which may or may not match the others) and lightweight virtualisation, where a single operating system kernel is used across all virtualised environments, but they are isolated from each other with separate networking, and process space. In this setup, memory is often shared across each virtualised environment, but the memory for each is isolated so you can't overwrite another machine's memory.
Lightweight virtualisation has taken a while to get going, but OpenVZ on Linux works reasonably well, and linux-vserver is another Linux option. Solaris containers (zones) are a standard part of Solaris 10 and higher (including Opensolaris) on SPARC and x86, and are very simple to set up and use if you have an existing solaris 10 install. Branded zones can also be used to emulate earlier solaris levels for improved backwards compatibility, and have basic support for Linux applications too on x86. AIX6 has workload partitions (WPARs) which provide similar functionality, with the interesting addition of live partiion mobility in the first release, which allows you to migrate virtual machines dynamically between systems.
Full virtualisation options exist on most platforms. VMware is one example, but on x86 you also have Xen (one of the first to make use of the hardware assistance in Intel and AMDs newer chips, and also paravirtualisation where the 'guest' operating system knows it's being virtualised and so can improve performance ) and VirtualBox is another good x86 desktop virtualisation system that supports running on a wider range of operating systems than the others. Versions of Xen are also included in Solaris 11 and above as 'xVM' and in Oracle's linux as OracleVM. On linux, kvm is integrated into the main kernel now. If you're running a windows desktop, why not try colinux or andlinux to run Linux (Debian or Ubuntu respectively) under windows in a seamless fashion?
Moving away from x86, you have LPARs (Logical Partitions) on IBM POWER5 and above, LDoms on Sun SPARC Niagara systems, and various options on HP-UX, including nPAR, vPAR, and Integrity VMs, which provide different levels of full machine virtualisation, depending on how much hardware isolation you want, and whether the management is done via a dedicated HP-UX install or not.
So there are plenty of options in the virtualisation space, and hopefully you now understand that there really is more to it than VMWare!