Image: via Notcot
I could have used this the other day.  After walking around feeling like a gaoler for months, I decided to purge my keys and get rid of ones I no longer use.  This gadget would have made the whole process a lot easier and less painful.  Called the Free Key and designed by Eric von Schoultz, it pretty much does what it says on the box.  Pressing the lock icon leverages open the end and lets you slip the keys on and off as needed.