Amazon recently unveiled yet another infrastructure building block to their suite of services. Simple Notification Service (SNS) allows you to build applications with publish/subscribe messaging functionality. SNS is distributed and redundant and retries message delivery if necessary. Owners create and manage Topics (URIs identifying specific topics, content or event types) that are received by Subscriber clients (applications, end-users, servers). The delivery of messages are guaranteed by the Publishers that send messages to topics. SNS in turn matches all of the topics to the list of subscribers interested in them and delivers the message to each one of them.
The SNS API is simple but also proprietary. If you have something running in JMS it shouldn't be a big leap but you are locked into AWS. The service currently support HTTP, HTTPS, EMail and Email-JSON protocols but more are on the road map. They are also working on a point-and-click interface for the AWS Management Console.
There is no charge for the first 100,000 SNS requests per month, the first 100,000 HTTP/HTTPS pushes and the first 1,000 email deliveries. After that it's $0.06 per 100,000 messages published, $0.06 per 100,000 HTTP message sent and $2.00 per 100,000 Email/Email-JSON notifications.
Since Force.com was not designed as a messaging platform, perhaps SNS would fit into an external integration landscape? Interesting possibilities.
The SNS documentation is available here.