The Pee Chee. I never had one and always wondered what they were (from all those Judy Blume books I read in middle school). Kinda like a precursor to the hallowed Trapper Keeper, the Pee Chee was a classroom staple, and lent itself to some awesomely creative modifications. I love how the lead runner on the bottom is able to take on so many personas – afro guy, hair metal dude (I saw a lot of those), and a couple that I felt couldn't be re-posted here. ;) Above are some of my faves.
Top left: O.G. Pee Chee cover; top right: by Marcia Rivero Bourne, 1967; bottom left: by Steven Leone; bottom right: by Scott Davis, circa 1983.