Poul Nielsen, a personal trainer, has created these awesome elastic business cards. You can't read the card unless you stretch the elastic. Then, not only will you be stretching your hands, and getting a little workout, but you'll see the contact info that will lead you to a bigger, more in-depth workout.
This is a brilliant example of experiential advertising, in that you are actually performing an action related to the product. This ties the business card to the company's value proposition while at the same time being completely memorable.