This can opener is referred to as a church key.
church key definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
Slang a device for opening cans, esp. originally beer cans, by punching a -shaped hole in the top
church key definition by American Heritage Dictionary
A can or bottle opener having a usually triangular head.