I am such an advertising specialties dork, that I have a favorite promotional product, and it's the KeyZee.
I've been carrying a KeyZee on my keychain for years, and there have been so many times that I was happy it was there. When I cat-sit and dog-sit for my friends (I love pet sitting!), of course I'm usually given a key to their house. With the KeyZee, I can easily add it to my key ring. When they get return from vacation, I can easily take it off to give it back. When I take my car in, I can take my car key off without giving the auto shop my entire key ring.
I also love how small and lightweight it is. It fits nice on my key ring without adding bulk and easily stacks with my keys. As a marketing tool, it's slim and easy to mail. The vibrant full color imprint makes the design possibilities limitless. It comes polybagged with a key ring and instruction card. These would be great to keep on the front counter of any business that deals with keys, and an inexpensive way to show your customers that you care.