This is pretty cutting edge for developing web-based applications: Opera 9 supports Server-Sent Events [whatwg.org] from the WHATWG Web Applications 1.0 specification.

Traditionally, when building an Ajax application, the browser continually polls the server, sending requests to the server, asking to get data back, making new HTTP requests for every single poll, putting more strain on the server than needed.

The event streaming approach [in Opera 9] instead opens a persistent connection to the server, sending data to the client when new information is available, eliminating the need for continuous polling.. the server no longer has to handle the overhead associated with clients asking for new data.

[my.opera.com...]