TenCent's WeChat (Android, iOS) might be the biggest messaging app you've never heard of. With 438 million users (more than 70 million outside of China), WeChat is dominating presence in the Chinese mobile Web and beyond. WeChat provides users with free mobile instant messaging, video and voice calls, group chat, and multimedia messaging (images, video, audio, stickers, etc). The app also includes quirky features such as "Friend Radar," "People Nearby" and "Shake" to quickly find new people to chat with nearby. Android Wear and Apple Watch apps are also available.