FutureMac

Matt Tucker writes on Jivesoftware.com about using XMPP (also known as Jabber) for push-based cloud services. The idea is that, rather than having the client software poll the service at a regular interval, which wastes resources, the server will push updates to the client as they become available. While this has always been the desired way to update clients, with HTTP it is not easy to set a push system up. 

Using the XMPP protocol, originally developed for instant messaging, one could develop an e-mail client that is set up as a subscriber in an XMPP connection. The server would send new e-mails to the client as XML fragments in the data stream.

A number of XMPP-related projects are hosted at Ignite Realtime. These projects include OpenFire, an XMPP-based IM server, Spark, an XMPP-based IM client, the Smack API, a Java XMPP API, and the XIFF API, a Flash XMPP API. Java.net has an interesting write up (including examples) of Smack.