Amazon SQS - Amazon Simple Queue Service is a fully managed message queuing service.
RabbitMQ - RabbitMQ is an open source message broker software.
Iron.io - Iron.io is the leader in cloud application services. IronMQ is a highly available message queue solution and IronWorker is a powerful async task processing service.
CloudAMQP - CloudAMQP automates every part of setup, running and scaling of RabbitMQ clusters. Available on all major cloud and application platforms.
Apache Kafka - Apache Kafka is an open-source message broker project developed by the Apache Software Foundation written in Scala.
Gearman - Gearman provides a generic application framework to farm out work to other machines or processes that are better suited to do the work.