DIY Maven on Curbly has a bunch of great ideas for alternative ways to use nail polish.
Given that the use of nail polish is "to be painted on things" -- it's a low-toxicity enamel paint -- I don't know if these count as "new" uses per se. My mother has been doing a few of them for years, particularly the "polish on the keys" trick, and she's far from alone. But they're all ideas that are useful, and maybe they're new to you!
Read more about them, as well as a few new nail polish tips from us, after the break.
To follow all the recommendations, you'll need to have at least two bottles: one clear, one with color. Use the clear when you want to hide or seal something; use the color when you want to mark something. If you'd like more precision for marking, consider using the kind of nail polish made for "nail art", which has a thinner and more precise brush. (Or, try using a toothpick tip instead of the polish brush.)
Make sure the surfaces on which you're using the polish are squeaky clean. In some of the cases described in the article, like sealing screws, marking keys, and noting water temperatures, all of which involve painting onto metal, you might want to go over the surface first with nail polish remover, to clean it thoroughly. Don't ever use acetone nail polish remover on plastic, though: the plastic will melt. In the case of the nick in the floor, you might want to lightly sand the area and use a tack cloth (or at least wipe over it with a damp paper towel to remove sanding residue). If you're putting clear nail polish over a wart, wash and dry the wart first.
Some Curbly readers have left some of their own good ideas in the comments: you can also use nail polish to label toothbrush handles (helpful on family vacations where half a dozen people or more may share a single bathroom), and you can use it to repair the tips on bobby pins.
I'd go one further than that: you can use nail polish to decorate the top of any metal barrette. If you'd like to make over a candy tin as a craft project, you can also use nail polish to paint on the metal (many other paints will simply bead up -- and no, don't do this if you still have some candy in the tin). If you need to thread a needle and the thread or yarn is being feisty, try coating the end of the yarn in nail polish. Clear is preferable, but since you'll be snipping off the thread end anyway, it doesn't really matter what color you use.
And if you ever want to give yourself a manicure or pedicure, you're covered.