RabbitMQ - RabbitMQ is an open source message broker software.
Google App Engine - A powerful platform to build web and mobile apps that scale automatically.
ZeroMQ - ZeroMQ is a high-performance asynchronous messaging library.
Dokku - Docker powered mini-Heroku in around 100 lines of Bash
Apache ActiveMQ - Apache ActiveMQ is an open source messaging and integration patterns server.
Google Cloud Functions - A serverless platform for building event-based microservices.