Zulip is the only modern team chat app that is designed for both live and asynchronous conversations. Zulip topics create a separate space for each discussion, so different conversations will never get in each other's way.
- - Zulip is the most popular Web-based, Windows, Android & Mac alternative to Spika.
- - Zulip is the most popular Open Source & free alternative to Spika.
Technitium Mesh is a secure, anonymous, peer-to-peer (p2p), open source instant messenger designed to provide end-to-end encryption. Primary aim of developing this instant messenger is to provide privacy which is achieved using cryptography and anonymity using Tor network.
The project seems no longer developed. Last version, released in January 2017, can be still downloaded from GitHub