ThinkGeek has an interesting product: the Retreev Compact Grappling Tool. A grappling hook can be surprisingly handy when you're trying to get things out of a tree, like a frisbee for the tenth time that day. The Retreev is perfect for this.
It's compact, built with a magnetic head made from a rare earth magnet, as well as stainless steel spikes that are stored inside the body of the hook, only to be assembled when you're ready for action. The magnetic head helps to retrieve metallic objects, but is still surprisingly light. You can attach the grappling hook to your keychain, so you're always ready to spring into action.
The hook is small and compact, about the size of a pocket knife.