Yik Yak is an app that functions like an anonymous, location-based Twitter, and has been dogged by controversy since its inception. Some colleges have tried to ban it, alleging that its anonymity facilitates hate speech. But founders Tyler Droll and Stephen "Brooks" Buffington insist that the app's main purpose is building a local community.
Price: Free (iOS, Android)
Previous reporting by Maya Kosoff, Jillian D'Onfro, Steven Tweedie, Alex Heath, and Lisa Eadicicco.