Rocket.Chat is an MIT licensed open-source communication application. Its features include (but are not limited to) live chat, video conference, screensharing, file uploads, role-based permissions, native apps, internationalized UI, rich media, notifications, full-text and regex search within history, emojis and much more. Rocket.Chat currently supports importing data from Slack and Hipchat.
BuddyPress is an open-source social networking webapplication that helps you run any kind of social network on the website, with member profiles, activity streams, user groups, messaging, and more. This plugin transform WordPress to into a social network platform. BuddyPress is designed to allow schools, companies, sports teams, or any other niche community to start their own social network or communication tool.
Nextcloud is a suite of client-server software for creating and using file hosting services. Nextcloud application functionally is similar to Dropbox. Unlike Dropbox Nextcloud does not offer off-premises file storage hosting. Nextcloud is free and open-source, which means that anyone is allowed to install and operate it on their own private server devices.
Lichess is an Internet chess server. Anyone can play anonymously, although players may register an account on the site to play rated games. All features are available for free, as the site is funded by donations. For registered players, Lichess employs a rating system, and grants the ability to compete in tournaments, post in the forums, and request a server-side full game analysis for any finalized game.