Campfire!

Mattermost is an open source, self-hosted Slack alternative.

Trillian is an instant messaging app that works on phones, tablets, and desktops while keeping your chats neatly synchronized between all of them! We think conversation should be easily shared between all of your devices, not trapped as text messages on your phone. Here's how we do it:• INSTANT MESSAGING: Your unique Trillian username can be used to send unlimited messages free of charge. Trillian uses your existing data plan and/or WiFi.• ANYWHERE: Trillian's continuous client feature delivers your chats instantly between all of your connected devices. In addition, Trillian automatically silences push notifications on mobile devices if you're active on a desktop - no more dealing with a phone buzzing on your desk all day!• SIMPLE: Trillian places a strong emphasis on fast and reliable message delivery and a beautiful, easy-to-use interface.• SECURE: All communication with Trillian servers uses TLS to ensure third-parties can't read your chats.• INTEROPERABLE: Of course, Trillian wouldn't be Trillian without support for Facebook Chat, MSN, Google Talk, AIM, ICQ, Yahoo!, and Jabber. »

HipChat is an instant messaging service designed for companies and teams. It has persistent chat rooms, one-to-one chat, searchable chat history, file sharing via drag & drop and clipboard, inline image previews, an API, chat over SMS, and more.

Gitter is a chat and collaboration tool for developers and users. It's a place for developers to talk.

Zulip is a group chat app.

hack.chat is a minimal, distraction-free chat application.Channels are created and joined by going to https://hack.chat/?your-channel . There are no channel lists, so a secret channel name can be used for private discussions.

dialog app is built and designed for corporate communication on all types of mobile devices - tablet, mobile phones etc.

Open Source private chat server. Your own private Chat server in minutes, plus additional features. No credit card required.A fast, secure and stable solution based on Rails.

ScudCloud is a non official open-source Ubuntu desktop client for Slack.

qiscus is a messaging system designed to mimic working environments by allowing users to structure their teams or projects in room-based discussion system thereby removing the need for other communication tools such as emails, messaging or video chat.