Ricochet is an interesting project too. It also doesn't have centralized servers. It routes messages through the tor network.

https://ricochet.im/

The developers aren't giving guarantees about it though. From the website:

Ricochet is an experiment. Security and anonymity are difficult topics, and you >should carefully evaluate your risks and exposure with any software.

We’re working on auditing, reviewing, and always improving Ricochet (and we’d >love more help). There will be problems. We hope to do better than most, but >please, don’t risk your safety any more than necessary.