GNU/GPL: 4 fundamental freedoms (1989 ... )  
  Freedom 0 :
The freedom to run the program, for any purpose  
  Freedom 1 :
The freedom to study how the program works  
  Freedom 2 :
The freedom to redistribute copies  
  Freedom 3 :
The freedom to improve the program, and release your improvements to the public  
  ⇒ Limit on program price  
  ! Proprietary software !  
  ! Open software !