The best alternative to Beeper is Ferdium, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Beeper and three of them are so hopefully you can find a suitable replacement. Other interesting alternatives to Beeper are Franz and Konnektor.
Open source cross-platform desktop app uniting messaging, email, productivity, and custom services in a side-by-side interface with easy account-free use, workspaces, built-in Todo panel, resource hibernation, robust privacy controls, and optional multi-device sync.
Centralize multiple chat and messaging services in one customizable interface, organize workspaces, receive synchronized notifications, manage multiple accounts per service, and easily access popular platforms on Mac, Windows, and Linux. Privacy controls included.
Konnektor is the developer-friendly alternative to Sameroom. It allows you to do the same but in a different manner. Currently you can use Konnektor for free if you deploy it on your own host.
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It does not use Matrix and it does not aggregate platforms into a single feed