Hopefully, you'll never be caught without a can opener when you need one. If you're out camping, however, this video shows you how to open a can with no tools at all.
The video is in Russian, but you don't really need to understand his instructions to follow along. Essentially, you put the can between your hands and press both ends on the edge towards each other until you fold a large, pointed dent in one side of the can. Flip it over, and repeat the process until the can has an hourglass shape, with sharp edges on the sides of the dent.
From the tone (and the setting) of the video, it's clear this is a survival tip, not something you'd use the next time you want to grab a can of soup from the pantry.