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.