If you thought the phrase "useful phone charm" was inherently oxymoronic, please meet Plug It's phone charm offering. It looks like a hunk of plastic junk, until you split it open to reveal a complete data cable.
As you can generally charge your phone via your data cable, the Plug It phone charm is fantastically useful. Retailing at only €7.95 ($12), most of the major phone brands are supported, excluding Sony Ericsson, who will be getting some USB charm love in the coming months. The Plug It charms are available in various colors and designs, and whatever you do, avoid the awful company website like the plague. (Either that or be forced to listen to their theme tune; #Plug it in, plug it in, baby!# My ears feel abused.) [Product Page]