* Allow your IPC (Industrial Production Certificate) supply to get over 250. 
  The game will sometimes give you free units. It will most commonly happen 
  when over 450, but still may happen at times if the total is between 251 
  and 449.

* Build so many units in the game until the IPC window freezes. The game 
  will give you free units the rest of the game.

* Go to "Options" then "Edit Unit". Set all units to the price of one IPC 
  then start the game. In the weapons development screen, get the Industrial
  Complex Development. Once you have it, everything is free, even if it says
  that it costs one IPC.