Being a frequent user of iPods and portable music players, I love this extremely simple, yet absolutely helpful design by Jaehyung Hong that secures your headphone wires and keeps them out of your way.
Whether you are eating, typing or running, the "Wire Button" makes sure those long wires stay at bay by clipping them to a button. Naturally, this won't help if you are wearing a t-shirt with no buttons.
I love it, so simple and cheap to produce yet so useful. I think Apple should pick it up and start including it with the accessories that come with the set.
Here are two other useful buttons: