I happened to catch this interesting hack on core77 and tried it for myself. You take four business cards, cut them up to interlock, leaving enough space for your earbuds to nestle inside, pop it open a bit and voila -- instant speaker cones for the tiny music makers. My MacBook Pro's headphone port recently became stuck in the line-out mode (the Cylon light is glowing and yes, I've tried to slide the little switch), and until I can get it to a shop I'm using these as my "speakers." Obviously the primary use would be for iPods, or anything without a built-in speaker.
Does it work? Hey, it's better than nothing. But don't expect to hear much if the noise in your area is anything north of murmur. The decibel boost is negligible, and the amplification is somewhat directional. It works well if you have an ample belly to rest the speakers upon when lying on the couch, as I do. A neat exercise in creative thought, all the same. If anyone can figure out the exact pattern the designer used, post a link in the comments.
I snapped some pics of me trying to replicate the design. I wound up altering it a bit, although my final speaker cone locks the headphones in quite firmly and can stand up in a variety of ways. It is narrower than the original, which doesn't help amplification. Check the gallery for details. Oh, and be sure to check out the neat iPhone earbud winder made from paper.