Google Cloud Messaging for Android
Google Cloud Messaging (GCM) is a service that lets developers send short message data to their users on Android devices, without needing a proprietary sync solution.
GCM handles all the details of queuing messages and delivering them efficiently to the targeted Android devices. It supports message multicasting and can reach up to 1000 connected devices simultaneously with a single request. It also supports message payloads, which means that in addition to sending tickle messages to an app on the device, developers can send up to 4K of data.
Google Cloud Messaging is completely free for all developers and sign-up is easy. See the Google Cloud Messaging page for registration, downloads, and documentation.