Via ezinearticles, here’s a list of the (dead link) top 5 reasons unconventional business cards work.
One of the best business cards I’ve ever seen was for a software company that based its technology on mathematics related to cubes. When you pressed down on the sides of the business card, the die cuts helped the paper fold into a cube. This reinforced the idea and gave people a reason to hang on to the card, perhaps to show their friends. In fact, I only recycled the card a few weeks ago, even though I met with their marketing department around 2002.