Slack is described as 'Facilitates teams' real-time collaboration with versatile chat interface and advanced search in configurable channels across multiple platforms' and is a leading Group Chat app in the social & communications category. There are more than 100 alternatives to Slack for a variety of platforms, including Web-based, Android, iPhone, Windows and Mac apps. The best Slack alternative is Element. It's not free, so if you're looking for a free alternative, you could try Element or Zulip. Other great apps like Slack are Rocket.Chat, Discord, Matrix and Trello.
Markee is a simple, secure and brandable alternative to Slack and Zoom. With Markee you can host video meetings, chats, and share files with anyone you choose — all from your browser.
With Instaforum, you can build a forum website that's as simple and usable as a modern messaging app. It can be public or private, and users don't have to install anything. It's easy to set up, runs on your domain with your branding, and you control your data.
A bird's eye view of everything that matters to you, be it your personal to-do's, team tasks or projects. As a project monitoring tool, Zapty also allows organizing and planning around more sophisticated projects, sharing, proofing, collaborating, and managing documents...
Team communication without the noise and distraction of instant messaging. Messages are sent on a fixed two hour schedule, giving plenty of time to get into deep work and flow.
SadnessChat SadnessChat is a free, real-time chat platform built for real conversations. Whether you're hanging out with friends or building a community, SadLounge has everything you need.
Colibri is an open source chat platform built on the AT protocol for communities big and small. It gives you the ability to create communities, manage members, and communicate through text, voice, and forum channels, all while maintaining true ownership of your data.
Twake Chat is a secure, open source messaging platform designed for enterprises. It offers seamless integration of chat, video conferencing, file sharing, and email, and more.
AstraChat is a standalone and lightweight Jabber (XMPP) client application for mobile and desktop that connects directly to your XMPP server without any proxies in-between.
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I like about Ziggs.io that you don't have to register and much faster than any other platforms since they don't store the messages.