Remarkably easy setup, automatic syncing with your phone's contacts and a feature-rich, ad-free experience all combine to make WhatsApp (Android, iOS) a wildly popular mobile messaging app. Users can send text, photos, voice and short video messages to their WhatsApp contacts, and the app has continuously added new features to its toolkit, such as emoji, and most recently, fully encrypted messaging between WhatsApp users. The app does require phone functionality, so it won't work on all devices. While WhatsApp used to have a subscription fee, it's now gone completely free, though this has led to some concerns about privacy and data sharing with third parties.
Viber (Android, iOS) is easy to set up and still offers a variety of options. Viber uses your phone number as your login, and the app syncs with your phone to help you find Viber-using contacts. You can use this app to send text, stickers and emoticons, photos, voice and video messages for free to other Viber users. The Viber Out feature lets you call non-Viber numbers and landline phones worldwide (for a fee). In addition, Viber's also introduced encrypted messaging features to the app, with a color-coded icon indicating chat security and encryption. You will need a SIM card and phone functionality to set up Viber.
Not content to rest on its laurels, Google has rolled out another messaging app that adds AI assistant functions to your everyday chat and SMS app. Google Allo (Android, iOS) has your typical mobile messaging features such as chat, photo messaging, stickers, emoji, and a doodle/annotation tool for photos, but the standout addition is its use of Google Assistant, the AI-powered assistant that can suggest quick message replies or find information relevant to your chat conversations, such as nearby restaurants, the weather report, and more. Additionally, there's an Incognito chat mode that features end-to-end encryption, private notifications and self-destructing messages.
Facebook's own foray into the mobile messaging field is Facebook Messenger (Android, iOS), a dedicated messaging app that builds on Facebook's existing chat network and adds more mobile messaging features. Users sign in via Facebook and can send other users the usual chat messages, as well as voice messages and voice calls to other Messenger users. On Android, Messenger has a neat "Chat Heads" feature that pops up in the screen to allow you to chat even when other apps are on-screen. Facebook also offers a slimmed down Messenger Lite intended for older Android devices that offers a slimmed down core messaging experience for older phones with less memory and processing power. And Facebook's planning on adding chat extensions that promise to let you bring your favorite apps into Messenger.
A mobile messaging app for the post-Snowden age, Telegram (Android, iOS) is aimed squarely at the security conscious user. Telegram features fast, encrypted chat messaging, with client-server encryption for standard chats. A Secure Chat mode provides end-to-end encryption so that only you and your intended recipient can read it. You can even set messages to self-destruct. You can share videos, documents and participate in group chats of up to 200 users.
Open Whisper Systems' Signal (Android, iOS) is another fantastic messaging solution for security-conscious mobile users. Folding in both of Open Whisper Systems' private messaging and calling apps RedPhone and TextSecure, Signal is an all-in-one messaging and voice call solution that uses end-to-end military grade encryption. You can send text, voice, group messages, media and attachments. Ease of use and strong, open source, regularly audited encryption makes it a favorite of the security conscious, with accolades from the likes of Edward Snowden and other privacy advocates.
With more than 697 million users, WeChat (Android, iOS) dominates in the Chinese mobile Web and is making a serious push for global reach. 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.