Adobe's AIR platform lets users on multiple computing platforms (Windows, Mac, Linux) use the same applications. Because the runtime platform is identical, developers can focus on making one version of that product, and making it well.
Since launching to developers in early 2007, there are now more than 500 applications for the runtime, many of which go in tandem with existing Web services. AIR has also allowed developers to make hybrid applications, or desktop tools that can integrate with various Web services, while still allowing some offline functionality that Web browsers don't yet have. This includes things like taskbar and dock notifications.
AIR is currently in its first major version, and continues to be developed alongside advances to Adobe's other Web plug-ins.