This is very basic trivia for a movie geek of my generation, but it's a story that's fascinated me for about 25 years. Apparently the story goes like this: Mr. Spielberg, hard at work on bringing E.T. the Extra-Terrestrial to the screen, approached the Mars company and requested permission to use their famous M&Ms candy as the alien's favorite treat. Universal wasn't willing to show them the final script, and so Mars passed. Spielberg's gang then went to Hershey with the idea of using their also-famous Hershey Kiss candies. But some smart guy at Hershey proposed ... yep, Reese's Pieces.
Keep in mind that this was 1981, and Reese's Pieces were still a brand-new product. Nobody seems to know for sure how many Reese's Pieces E.T. helped to sell, but I'm guessing it's in the billions. Bad move for Mars. Great deal for Hershey. But in the long run ... wasn't this really bad news for the moviegoer? After seeing what E.T. did for those new little peanut butter candies, hundreds of companies were about to BEG to have their products featured in films. So if the prevalence of 'product placement' really irks you sometimes, you can throw a little blame on silly little Elliott. Maybe he should have laid a trail of popcorn down the hallway. Aliens love popcorn!
Resources: IMDb, Wikipedia, and this MSNBC article. And thanks to Cracked.com for the pic!