I don't know about you, but I have a stack of keys that would make the maintenance man at the Empire State Building blush. Professional Organizer Beth Ziegler posted an awesome tip on how to keep keys organized on her blog Bneato last month that was picked up by Lifehacker.
[Image from Lifehacker]
Give your keys a nice shiny coat of nail polish to tell which belongs to which door, lock, or mystery trunk. If you have a particularly old key, you might try cleaning it with nail polish first. It'll take off any oil or grime that's on it and help the polish stick better.
Sidenote… Beth Ziegler = awesome. She writes for the equally awesome Apartment Therapy and I've added her RSS feed to my list (though I had a hard time deciding if I should file her under organization or cool design…hmmm).