Spika is described as 'is the world's first open source mobile messenger solution. This solution includes an iOS App, an Android App, and a server backend with an admin panel' and is an app in the Social & Communications category. There are four alternatives to Spika for a variety of platforms, including Android, Android Tablet, Linux, Self-Hosted solutions and Windows. The best alternative is Zulip. It's not free, so if you're looking for a free alternative, you could try Bit Chat or Cyanide.
Technitium Bit Chat is a secure, peer-to-peer (p2p), open source instant messenger designed to provide end-to-end encryption. It can be used over Internet and private LAN networks for instant messaging and file transfer.
Chat with people you know, or find new friends. Main Features: Share messages, photos, videos, sound/voice recordings and location. Notification of messages when application is not running. Profile picture and about information.