Virtualization (the ability to run multiple OS simultaneously) is gaining lot of traction nowadays. In the PC world this started initially with VMWare and Virtual PC (which Microsoft acquired from Connectix) for development and testing purposes soon gained popularity in the servers. In servers virtualization is used to consolidate servers and applications into fewer servers and also used for running legacy OS and applications.

Today the entry barrier is greatly removed for Virtualization software with many of them available free (as in free beer), following is a partial list of them.

1. Microsoft Virtual PC 2007 for desktops
2. Microsoft Virtual Server 2005 R2 for servers
3. VMWare Player for desktops
4. VMWare Server for servers

Recently few more has joined the list, they are:

1. VMWare ESXi - A hypervisor that allows you to run production applications at near-native performance is now free
2. Sun Microsystems' Virtual Box - A popular desktop virtualization software that Sun Microsystems recently acquired from Innotek and turned into an Open Source GPL software
3. Microsoft recently released its Server class production ready Hypervisor product called Hyper-V that is going to be part of Windows Server 2008 for a nominal fee of $28.