Flowdock!

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

Basecamp is a web-based project collaboration tool that allows groups of people to share files, meet deadlines, assign tasks, and centralize feedback.

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.

Quire is a new-generation collaborative task management tool for organizing tasks in a tree-like structure wrapped in simple and user-friendly UI.

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

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.

Campfire is a simple web-based real-time group chat tool for business. It makes real-time communication with 2-60 people as simple as visiting a web page.

Aha! is cloud-based software to set your product strategy based on market dynamics. Track imperatives through releases and features.

Echoplexus is an anonymous, web-based, IRC-like chatting platform that makes its best effort to respect your privacy. It allows you to create public or private channels. You can encrypt your chats. You can secure a pseudonym for linkable anonymity. You can code and draw together in real time. You can make free and secure Peer2Peer video and voice calls with the people in your channel using WebRTC.

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.

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.