To be part of the in-crowd in Silicon Valley, you have to know about and use the latest hot new apps. Years ago, that was Gmail; this week it's Hey, a new email service. Below is a list of six of the hottest apps in the Valley. Fifteen years ago, it was Gmail, Google's now super-popular email site that started as an invitation-only service. But Hey's not the only hot new app to set the Valley a buzzing.