Since its inception, the Web has been synonymous with the browser. Pundits hailed NCSA Mosaic as "the killer app of the Internet" in 1993, and today's browsers share an unbroken lineage from that humble beginning.

Today's Web sites are another matter, however. Gone are the static pages and limited graphics of 15 years ago. In their place are lush, highly interactive experiences, as visually rich as any desktop application. The Web has become the preferred platform for enterprise application delivery, to say nothing of online entertainment and social software. In response, new kinds of online experiences have begun to emerge, challenging old notions of what it means to browse the Web.

Take Twhirl, a desktop client for the Twitter online service. Double-click its icon and the application launches in seconds. Its window is small and stylized, with an attractive, irregular border and configurable color schemes. What few controls it has are convenient and easy to use. It's sleek, fast, and unobtrusive. In short, it's everything that navigating to the Twitter Web site with a browser is not.

But don't be fooled. Although it looks and feels like an ordinary desktop application, Twhirl's UI is rendered with HTML, CSS, Flash, and ActionScript. Essentially, it's a Web app.

Twhirl is built on Adobe AIR, which has a lightweight client library that allows Web developers to use familiar tools and languages to build first-class desktop applications. Software created with AIR is fully interactive and network-enabled, with a rich UI. But unlike traditional Web applications, AIR apps gain the immediacy and user engagement that come from running outside the browser window.

[ For an in-depth look at rich Web frameworks, see our reviews of Adobe AIR, Microsoft Silverlight, Curl, WaveMaker Visual Ajax, open source AJAX toolkits, and other rich Web development tools. ]

"The browser is terrific for transient experiences … things that a user might do once in a while, or for a short amount of time," says Ed Rowe, director of AIR engineering at Adobe. A frequently accessed service like Twitter, on the other hand, cries out for a lightweight client. AIR allows the same developers to build both.

But AIR is only one branch in the Web's ongoing evolution. Already, Google, NetSuite, Salesforce.com, Zoho, and others are using Web tools and infrastructure to deliver full-fledged enterprise software, defying the limitations of today's browsers. As the static Web gives way to RIAs (rich Internet applications), client software must continue to adapt and evolve; and in some cases, this could very well mean stepping beyond the traditional browser altogether.