Today, nearly every modern computing-related company uses directly or indirectly a virtualization software. As seen with the recent Cloud computing trend, it is a sector that recently exploded in popularity and it is now accessible to everyone.
There are now a LOT of alternatives to virtualize an OS :
Isolation : OpenVZ, VServer, LXC, UML, BSD jails, Solaris zones…
Paravirtualization : Xen
Full virtualization : VMWare Player/Workstation/ESX/ESXi, KVM/Qemu, Virtualbox, Bochs…
When having to deal with a large pack of machines for building packages, testing software on multiple OSes or optimally use a powerful server with lots of RAM, you rapidly should, if not have to, realize that Virtual Machines can make your life significantly easier. It was the case for us.