Cream of tartar mixed with hydrogen peroxide to form a paste is an invaluable stain remover. It can be used on tubs to get rid of hard water and soap scum. I used to use the 'scrubbing bubbles' to combat our super-hard-water and my husband's affinity for Irish Spring. The can looks cute, but the smell gave me a headache, the chemical cocktail makes me cringe, the packaging is not recyclable, and it was never really thick enough to stay where needed on the side of the tub. Plus I always felt a little nervous bathing my daughter after using such a variety of toxic chemicals. You won't have any of those problems with cream of tartar and hydrogen peroxide. They're dirt cheap too. They're also perfect for removing that brownish-tinge my sink gets after tapping off my excess mineral makeup day after day. You can even use water mixed with cream of tartar to form a paste to remove a laundry stain. It all comes down to chemistry: you need an acid (like cream of tartar, lemon juice, or vinegar) combined with a base (water, baking soda, hydrogen peroxide) to create a chemical reaction. As the base molecules absorb the acid molecules they release a lot of hydrogen molecules. It's this process that dissolves a stain. Commercial cleaners aren't doing anything different to dissolve stains they are simply using more corrosive (harmful) acids, adding in a lot of unnecessary (and harmful) additives to preserve the mixture, add fragrance, make it foamy, and oh yeah, charging a lot more too. If you mix up the chemical reaction in a fresh batch you'll actually get quicker results. (And Mr. Ruehl my sophomore chemistry teacher, I really wish I'd paid more attention to you instead of mocking your goggles and playing with the Bunson burners).
Ants don't seem to like cream of tartar at all so if you get tired of picking your lemon peels up off the ground to prevent ants, try sprinkling cream of tartar around the outside of their ant hole. (I can't seem to make that sentence not sound a little bit dirty...)
And how about that tasty lemon meringue pie? Your poor meringue would never stand up without the stabilizing effects of cream of tartar! It turns out that you can even make your own baking powder by mixing baking soda and cream of tartar together.
And where does this little miracle acid come from? Well I'm glad you asked because it turns out that cream of tartar is a naturally occurring byproduct of fermenting grapes. Scrape out the wine casks and you have cream of tartar. Drop that little gem of trivia at your next wine-tasting party why don't you!