If you've ever used a computer and had it freeze or otherwise lock up, you may have had to pull the power cord, either literally or by holding the power button. But how do you do this for a virtual machine?
In the Virtual Machine menu, there are of course power commands: Shut Down Guest, Suspend Guest, and Restart Guest. However, these are what are known as soft power commands - they're requests to the guest OS, which it can ignore (or in the case of a deadlocked guest, might not be able to handle). In contrast, hard power options are not ignorable by the guest - it's the virtual equivalent of yanking the power cord. To get these hard power options, hold down the option key on the Virtual Machine menu to change the power commands to Power Off, Suspend, and Reset.
Obligatory note of caution: Just like with a physical computer, you risk corrupting the contents of the (virtual) disk if the guest OS isn't in a quiesced state.
Side note: Some virtual machines have the default options switched (i.e. hard power options by default, and pressing the option key changes to soft power options).
Bonus tip: Holding down the option key to get alternate menu items is used in other Mac applications - for example, try it in Finder.