The best open source alternative to AstraChat is Element. It's not free, so if you're looking for a free alternative, you could try Jitsi or Pidgin. If that doesn't suit you, our users have ranked more than 50 alternatives to AstraChat and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to AstraChat are Mattermost (Freemium, Open Source), Rocket.Chat (Freemium, Open Source), Zulip (Free Personal, Open Source) and ChatSecure (Free, Open Source).
Astian Spika is an instant messaging service focused on speed, security and productivity. With great differences such as integration with Astian Cloud, support for video calls, calls among other features.
Jabbin is an original mashup of Social Network, Instant messaging and VoIP developed entirely on the widely-used and open protocol, Jingle/XMPP, and open source software, including Asterisk, Openfire and Jobill, a billing system developed by us.
BeagleIM is a lightweight and powerful XMPP client developed by Tigase, Inc. It provides an easy way to start using the XMPP Protocol (formerly known as Jabber) if you've never used it before. Veterans of the protocol will find many features here they are familiar.
PrivMX is safe and easy collaboration software for all kinds of teams, with virtual workspace protected by end-to-end encryption. Each team uses desktop/laptop client software and its own (zero-knowledge) PrivMX Team Server.