Amazon have released Amazon Simple Notification Service to beta.
Amazon SNS provides developers with a highly scalable, flexible, and
cost-effective capability to publish messages from an application and
immediately deliver them to subscribers or other applications.
Amazon SNS delivers notifications to clients using a “push” mechanism
that eliminates the need to periodically check or “poll” for new
information and updates.
Amazon SNS can be leveraged to build highly reliable, event-driven
workflows and messaging applications without the need for complex
middleware and application management.
The potential uses for Amazon SNS include monitoring applications,
workflow systems, time-sensitive information updates, mobile
applications, and many others.
The first 100,000 notifications per month are free.
Read more on Amazon Web Services