Rubber bands. I have a bag at home I use mainly to keep decks of cards together (I play a lot of board games), but they're nice to have on hand for a variety of situations. Emily Callan over on Instructables has put together a list of ten uses you may not have thought of, from opening jars to keeping doors open.
The entire compilation is available here and covers a range of typical activities that might be a little less frustrating with the aid of an elastic band. One trick I wasn't familiar with was keeping doors open by covering the locking mechanism. It requires two handles and the appropriate type of door, but it's a very inventive use of the humble stationery staple (no pun intended).
Photo by Emily Callan
And of course, ammunition.
10 Life Hacks Using Rubber Bands [Instructables]