• Signal app and protocol (end-to-end encrypted by default, but centralized server, no federation)
• Matrix protocol (end-to-end encrypted, decentralized, federated, user gets to choose app to use)
• Delta Chat (I am not sure about this one, I know too little at the moment)
• Simplex Chat

Let's say you have a tablet or reserve phone without a phone number (i.e., SIM-card) that you would nonetheless like to have Signal installed on. Turns out it's possible.

BUT NOTE THAT SIGNAL WILL DE-REGISTER YOUR ORIGINAL DEVICE ONCE YOU REGISTER THE NEW ONE!
There is still no way to have Signal on more than one device, and you can't link Signal on one device to another device. Too bad, would have been nice, especially with database encrypted at rest (which Molly allows).

• Connect your secondary device to the Internet
• Install Molly-FOSS or Molly or Signal on the secondary device, and initiate Signal's registration process (enter your phone number)
• You will get a code via sms on your primary Signal device
• Enter the code on the secondary device using the numeric keyboard (this worked just fine in my case)
• Enter your Signal PIN
• With that, Molly-FOSS/Molly/Signal should start and show you your groups

While we're at it, why not install Molly or even Molly-FOSS instead of the official Signal app? The benefit of using Molly-FOSS is that we can install it on a device not using Google (the drawback is that push notifications will consume a little more battery).

• What originally sent me exploring Signal app forks was this issue
• https://github.com/mollyim/mollyim-android
• https://molly.im/fdroid/
• https://reddit.com/r/CalyxOS/comments/ptll4t/molly/
• Another Signal fork