VirtualBox is a pretty good virtual machine system from Sun Microsystems that has several key benefits:

• It's free, whereas most others are commercial and can cost a chunk of change,
• It's multiplatform, so a company can standardize on it across all of their supported systems.

It seems to be pretty capable in regards to running client operating systems - it has a menu which lets it automatically configure itself for a wide variety of Windows releases, UNIX distributions, etc.