A VM (a Virtual Machine, like VirtualBox, VMware, Virtual PC) pauses when the mouse is not within the window (or full screen in case using a virtual windowing manager like VirtuaWin or Dexpot). When the mouse enters the window it sends the resume hotkey to it. When it leaves, it sends the pause command to it. The number of VMs running will be limited only by RAM, not CPU. Includes and excludes by window title, because you don't want to pause a server VM.