I have a lot of nail polish. Some I wear, some I keep around for random projects or other uses. Here are some of my favorite unusual uses for nail polish.
Step 1: Prevent Tarnishing
A lot of the charms you find in beading stores are listed as silver, Tibetan silver, or Bali silver---this really just means plain, cheap metal alloy that WILL tarnish. I've found that coating these charms in clear nail polish extends their life. Adding a new coat every so often will further help to stave off unsightly tarnish.
You can also coat the inside of your rings to keep them from turning your finger green.
Step 2: Securing Screws
Nail polish can help secure loose screws or screws that are constantly wiggling out. Polish will also help prevent against rust/tarnishing.
Step 3: Cracks and Scratches
Polish can be used to fill in scrapes and scratches in wood furniture. Clear is probably your best bet as I'd imagine it'd be hard to match up wood coloring.
Clear or white nail polish can also be used for sealing chips in china. The polish can help prevent further damage by keeping water from getting to the chips and causing cracks.
Step 4: Color Coordination/Identification
Nail polish can help you easily identify keys, electronics cords, and more!
I've also used nail polish to write my initials inside staplers, hole punchers and other things to prevent them from going missing.
Step 5: Loose Buttons
Apply a coat of clear polish over the thread of a loose button to secure it.
Step 6: Labels
Apply clear nail polish to homemade labels to keep them from smudging or wearing off too quickly. Polish can also be water resistant, to a point. You will need to reseal every so often.
As previously mentioned, I'll also use colorful polishes to add my name to objects that tend to go missing in my classroom.
Step 7: Matches
Covering the tips of matches can help protect them against moisture, therefore extending their life (and not leaving you freezing in the woods when camping).
Step 8: Screens
Have a window screen with holes? Bugs starting to get in? Cover the holes with nail polish to prevent the screen from becoming more damaged and keep bugs out!
Step 9: Scuff Marks
Keep your shoes looking fresh by using matching nail polish to touch up any markings. If you can't finding matching colors, use nail polish to add a little flair and decorate your shoes.
Step 10: Pranks
Cover a bar of soap in clear nail polish to prank your friends :-D
This doesn't ruin the soap and after a few attempts at washing, the polish will wear off.