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.

The only minor problem I've found is in regards to how it handles disk images - the virtual disk files it creates to store the client operating system. The problem is that you can't just duplicate the file from your file manager or command line, because it identifies each disk file with a unique identifier that obviously doesn't change when you manually copy it. Instead it comes with a command which will do the duplication for you, only it's hidden in OSX.

So, if you need to duplicate a drive image in VirtualBox on your Mac OSX machine, you would run a command like so (all on one line):

/Applications/VirtualBox.app/Contents/MacOS/VBoxManage clonevdi Windows_XP_IE_6.vdi Windows_XP_IE_7.vdi

Ta-daa! All done.