I have a small box that I keep all of my gift cards and gift certificates in so I can find what I need when I need it. To my knowledge I have never lost a gift card, but according to Gift Card Advocate 10% of gift card face value never gets used. Because people “lose them, forget about them, forget to use them or have great difficulty using them” that adds up to $9 billion dollars that was not used in 2008.
If you are interested in selling, trading or cashing in your gift cards this site would be an excellent starting place. They link to over 30 sites that exchange your cards in one way or another and include site descriptions, how long they have been around and what types of cards they deal with.