Home building.
Not only can you save insane amounts of money, you gain a confidence in the result that you cannot get any other way. You know what's in them walls, it's no big deal to go back into them when you need to.
Often further savings are available by using salvage materiel, as you have the time and involvement necessary to separate the scrap from what can be used. You can't really skip this step with "new" anymore, anyway.
