The key ingredient - baking soda. Inexpensive, easy to find, extremely versatile and a household item most people already have at home.
The baking soda softens the water and rinses off the soap scum around the tub. It'll do the same for your hair and remove shampoo buildup.
Run a load of without clothes in your washing machine with baking soda and vinegar in place of detergent and it will clean out the buildup in the machine's agitator and tub.
My favorite use of baking soda - clogged toilets. I pour about 2/3 cup into the toilet, then 2-3 short squirts of dish detergent (original formula Dawn works the best), vinegar, and about 2 cups of hot water. Repeat application until the clog dissipates. Since finding this information on the internet, it hasn't failed me yet. No plunger, snake, or expensive visit from the plumber needed.
I know many experienced household cleaners have a bunch more uses for baking soda than I do. Like tissues, it's a must have in my house. If I'm down to a single box, it's off to the store for more.