Probably Google, or in broader terms, search engines (although I hate Bing and Yahoo as search engines). And if we want to get more specific, Wikipedia.
The number times I use my phone/tablet to look up information in school or on my couch is insane. Naturally I love learning, consequently it drives me nuts when I can't remember something I learned in school that day or a week ago (and unfortunately I forget a lot :/). So without fail I will immediately type in what I'm trying to remember or figure in to Google, which 99% of the time takes me to Wikipedia which is a valid source because it's always right despite what my teachers say so screw them I don't even care no more, and BAM I have what I'm looking for. Without search engines, I'd have to collect a vast library of knowledge, even more vast than that library in the desert in Avatar: The Last Airbender, to quench my appetite of remembering everything. And not only would I have to collect it, but I would have to sort through an incomprehensible amount of pages just to get one tiny piece of information that I'm trying to remember. And like I said, i forget a lot, so that would be time consuming and doesn't sound like a lot of fun.
So yea, thank God for Wikipedia.