At the grocery store recently, I saw, in the spice section, a white powder called "cream of tartar". What is this? What's it for? Why the funny name?

Potassium hydrogen tartrate, an acid salt. It comes from the sediment produced in making wine. (Yes, I looked it up!) I know you use it to add substance to whipped egg whites, and it apparently creates a creamier texture to things like frosting.

It adds volume to cake mixes and scones. Baking Soda + Cream of Tartar = Baking Powder.

Indispensable when you're making meringue.

Right, it's more like a stabilizer for delicate things like egg whites. I've never used it in whipped cream, but I imagine it would stablize that as well. It's nearly tasteless.