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Delta Chat: A messaging app that is completely compatible with the existing e-mail infrastructure. So, with Delta Chat you get the ease of well-known messengers with the reach of e-mail. Moreover, you're independent from other companies or services -- as your data are not related to Delta Chat, you won't even add new dependencies here.
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Koler: Uniquely stylized phone app with customizable features, designed with the user in mind. It uses swipes and minimalistic design, making everything more beautiful, intuitive, yet still productive.
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Mattermost: An open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams.
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QKSMS: An open source replacement to the stock messaging app on Android. Features a beautiful, intuitive, and clutter-free design that allows you to focus on what matters, while stil being customizable.
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Signal: A messaging app for simple private communication with friends. Signal uses your phone's data connection (WiFi/3G/4G) to communicate securely, optionally supports plain SMS/MMS to function as a unified messenger, and can also encrypt the stored messages on your phone.
