Another cool thing one could build on the moon is a rail cannon to bombard Earth. Moon's gravity well makes it relatively easy to hurl stuff from there to here,

Another cool thing one could build on the moon is a rail cannon to bombard Earth. Moon's gravity well makes it relatively easy to hurl stuff from there to here, and the stuff _accelerates_ as it approaches Earth, assisted by earth gravity. The scenario is explored in detail in Heinlein's "The Moon is a Harsh Mistress".