A command to fire up a virtual machine without the associated GUI that comes with it when started via the Oracle VM VirtualBox Manager.
Navigate to your VirtualBox install directory:
cd "\Program Files\Oracle\VirtualBox"
Then fire up a VM with:
VBoxManage startvm "Testing Server" --type headless
Update: Since VirtualBox 4.2, holding down shift whilst clicking Start starts the VM headless. Why this couldn't be put in the menu somewhere, I don't know!