The little hole in the side of the DS is nominally designed for the wrist strap, to keep your DS safe from slippery-hand syndrome. In addition, for the original DS, it provides access to the thumb-stick thing for Super Mario 64 DS.
However, that little indentation also provides another outlet for self-expression. There's no end to the junk you can dangle from your DS: cell phone charms, screen cleaners, extra styli, preorder-bonus stuff, homemade beadwork-- whatever you can imagine, you can tether it to the side of your console. Are you rocking any charms?