What are some alternatives?
When comparing twitter.github.io Kestrel and NATS, you can also consider the following products
RabbitMQ - RabbitMQ is an open source message broker software.
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
CloudAMQP - CloudAMQP automates every part of setup, running and scaling of RabbitMQ clusters. Available on all major cloud and application platforms.
Socket.io - Realtime application framework (Node.JS server)
Amazon SQS - Amazon Simple Queue Service is a fully managed message queuing service.
SignalR - SignalR is a server-side software system designed for writing scalable Internet applications, notably web servers.