OS X: Suddenly only one window/application visible at the same time
This answer lifted in its entirety from Super User : Snow Leopard desktop - I want two files from two different applications open at the same time. Won't let me


  Have you per chance turned on "Single App
  Mode"?
  This hidden preference causes Mac OS X to hide all applications but
  the current active one.
  
  You can tell by running the following command in Terminal.app:

defaults read com.apple.dock single-app

  
  If it prints a 0then it is not turned on. If it prints a 1 then
  yes, it is turned on.
  
  You can disable this trick by running:

defaults write com.apple.dock single-app 0