This is a decentralized messaging software that offers end-to-end encryption and a direct data channel between users. It is free and open source.
This project is cross-platform (.NET standard, C#), and its peer-to-peer (P2P) network doesn't put a limit on the size of the data you can transmit. Users can exchange huge files directly, without servers, using the 'UDP hole punching' technique. What's more, there is no link between user ID and phone number or email.
If you're interested in the technical details, you'll be glad to know that users are able to compile the messenger from source code, audit the source code themselves, and run the messenger on Linux, Windows, Android, or iOS. All in all, this messenger seems like a great choice for anyone who values privacy, security, and flexibility.
Version 1.1: Minor GUI improvements