Salesforce.com, Digg, and Twitter have adopted the Streaming API to make it possible to process data streams in near- real-time. With Force.com’s Streaming API, data transfer is:
What makes the API notable is the model used for data exchange. Rather than constantly polling the server (e.g. over HTTP), the Streaming API uses the Bayeux protocol. The Bayeux protocol allows the client to subscribe to an administrator-created ‘topic’ on the server. As a result, API calls aren’t wasted on loading unchanged data and it is easier to stay under the call limits of Force.com’s multi-tenant architecture.
More information on the API can be found in the Force.com Streaming API Developer’s guide pdf.