Adobe AIR is a cross-platform runtime environment developed by Adobe. AIR stands for Adobe Integrated Runtime and it allows for the creation of great Internet apps that can be executed on a variety of platforms, including Windows, Linux, and Mac OS X.
Adobe has a website full of published AIR apps which range from desktop FLV players, to clocks, converters, players, time, and resource management apps, among many, many others. One of the great benefits of writing an app in AIR is that it is instantly available to millions of users. Adobe AIR is probably already installed in a multitude of computers world-wide. There are some very popular AIR apps out there, like RichFLV, Adobe Media Player, Shrink-o-Matic, and WebKUT. But what is great about AIR is that anyone with the minimum coding skills can create rich apps.
Once you have Adobe AIR installed on your Mac, installing AIR apps is as easy as downloading the files and double-clicking on them or installing them from a supported web browser. Then, a new .app file will be added under your Applications folder and you can treat it as any other .app file. In my experience with AIR apps, I found that most of them are very responsive and show elegant designs. I don't think I have experienced any problems with them.
Make sure to check the AIR Marketplace to find very useful apps.