The app does require phone functionality, so it won't work on all devices.
While Whats App 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.
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.
Folding in both of Open Whisper Systems' private messaging and calling apps Red Phone and Text Secure, Signal is an all-in-one messaging and voice call solution that uses end-to-end military grade encryption.