Duct tape has extended the life of many a highschool backpack. I wish they'd stop forcing us to get clear or mesh backpacks. The old backpacks were much more durable. I've had my old Scooby-Doo backpack since the fifth grade and it only started tearing up in the last year or so, despite being in constant use. But that's neither here or there. We're discussing tape, not backpacks or stupid school rules.
Scotch tape is also very useful, especially for wrapping presents, so it comes in at a VERY close second. I mean, you wouldn't wanna give someone a gift wrapped with duct tape, would you? Well, if you're Red Green, maybe... ah well. XD