NATS.io is a simple, secure and high performance open source messaging system for cloud native applications, IoT messaging, and microservices architectures.

Celery is an asynchronous task queue/job queue based on distributed message passing. It is focused on real-time operation, but supports scheduling as well.

Scuttlebot is an open source peer-to-peer log store used as a database, identity provider, and messaging system. It has:

Global replication
File-synchronization
End-to-end encryption