title: Detecting an Application Focus Change / Hooking something for HWND changes?
body: <p>How could I detect when any application loses focus?  Is there any system hooks to use?  I want to pick up when ever a new application is in focus, or when one is lost.  Even having the "current" HWND of the user would be good.</p>
  
  <p>I am using C#, however, its all win api stuff I am sure... so any info would be great.</p>