It is free, very easy to use and supports Windows products, Linux and Mac.  It is more user-friendly than VMWare Workstation.

I've been running virtual machines on my Vista laptop for Windows XP Pro, Windows 7 Ultimate and Ubuntu (just for the heck of it) and I plan on installing more Virtual Boxes down the road (Windows 2008 Server, etc).

http://www.virtualbox.org/

I used to use VMWare Workstation but uninstalled it.  I couldn't see any advantage and Sun Virtual Box being free suited my budget more.

For a developer it is VERY useful to have it for testing purposes and to save money on equipment, etc.

Russ Brown (Using C#2010/SQL Server 2008)