Duct tape is possibly the most useful object commercially available today. In its traditional manifestation, it's a roll of grey, shiny adhesive tape, usually about two inches wide. It's very sticky and can be torn in more or less straight lines by hand. Duct tape has been used to fasten almost everything to almost anything else, quickly and cheaply1. For this reason it's a staple fix-it product on film sets and in theatres. In addition to its basic use as an adhesive tape, however, it has been used for a wide variety of other purposes in its time and you really can't go wrong with having a roll handy for emegencies.
Attaching one object to another. This is duct tape's traditional function. Because of its width and and powerful adhesive qualities, it's often used to tape wires or cables to the floor so that no-one trips on them.
Covering holes or fixing tears in a wind screen or mosquito netting.
Pasting pages of books back together again if the pages are printed only on one side or if you don't particularly care whether you can still read the back.
Repairing book binding.
Holding the sole of one's shoe to the shoe body should the two decide they wish to part company.
Fixing small holes in inflatable sporting or recreational equipment.
Creating a magnificent, slightly squishy hockey puck2.
When still on the roll, a very large bracelet for you.
Waterproofing your clothes. This requires an awful lot of tape, however, particularly if you are, ahem, big-boned.
A bandage for wounds. Be careful as taking the duct tape off is rather painful.
Writing big, silvery letters on banners.
Making a plastic wiffle-ball bat suitable for hitting relatively light balls very, very far. If you wrap a fairly significant amount of the tape around one of those skinny yellow plastic things, you can whop the tar out of a tennis ball.