Our small startup, Quizlet.com, is using online flashcards and games to make studying fun, social, and more productive. We've got 800,000+ registered users and over 50 million user-generated flash cards on the site.<p>Just wanted to share with HN a cool story...<p>We have an API that lets developers create apps that search and download from our big flashcard database so our students can study in new ways and when on-the-go through mobile apps.<p>One of our favorite iPhone Apps using the Quizlet API was Agilis Lab's Flashcard Touch ( http://www.agilislab.com/ ). It was a simple but beautiful implementation.<p>For the month of March, we decided to heavily promote their (previously $2.99) app if they'd make it free for the month. The app had only been getting a relatively small number of downloads and had ~16 reviews on Feb 28th. After only a few days of the promotion, the results have overwhelming:<p>- Day 1: it hit #2 free app in the Education category of the App Store and had 5,000 downloads<p>- Day 2: it hit #1 free app in Education category and made #94 in overall free apps<p>- Day 3: hit #40 in top free apps overall, then 33, and kept climbing<p>- Day 4: hit #15 in top free apps, (on the coveted Top 25 page), surpassing the Facebook app in the process. now has almost 500 reviews.<p>Of course the big question is how their app will do once the initial popularity fades and when it's no longer free, but it sure is fun seeing so many people get to try out the app and get exposed to Quizlet.<p>We'd really love to see other API developers do even more with Quizlet. We'll be releasing 2.0 of our API soon which will include the ability to get authenticate, get private sets, and even upload flashcards. http://quizlet.com/faqs/the-quizlet-flashcards-api/