Can you freeze chopped zucchini?
It is generally recommended to "blanch" vegetables before freezing them. Blanching is heating the vegetables (by boiling or steaming) for a specific number of minutes. You typically follow blanching immediately with an ice-water bath, to stop the cooking process. 

From The National Center for Home Food Preservation: 


  Blanching slows or stops the action of
  enzymes which cause loss of flavor,
  color and texture. Blanching cleanses
  the surface of dirt and organisms,
  brightens the color and helps retard
  loss of vitamins. Blanching also wilts
  or softens vegetables and makes them
  easier to pack.


For vegetable-specific instructions on blanching, see this list.