Hydrogen peroxide when swished in your mouth will whiten your teeth and works as a cheap disinfecting mouthwash.
Brushing your teeth with baking soda and water paste will whiten your teeth too.
A solution of 2 tsp. Tea Tree oil mixed with 2 cups water in a spray bottle, sprayed on and left, will eliminate mold spots in your shower for a month or so. Straight vinegar will do the same, most of the time.
Rust and hard water stains can be removed with full-strength lemon juice.