http://www.joelonsoftware.com/items/2007/09/18.html

So if history repeats itself, we can expect some standardization of Ajax user interfaces to happen in the same way we got Microsoft Windows. Somebody is going to write a compelling SDK that you can use to make powerful Ajax applications with common user interface elements that work together. And whichever SDK wins the most developer mindshare will have the same kind of competitive stronghold as Microsoft had with their Windows API.

In my opinion, the winners will be one of these or any other standards-based technology, but not any of the latest runtime-based environments, like Flex, Silverlight or AIR. These are just new ways to pronounce the word Java, and as Joel states, this sandboxing thing just does not work:

You can follow the p-code/Java model and build a little sandbox on top of the underlying system. But sandboxes are penalty boxes; they’re slow and they suck, which is why Java Applets are dead, dead, dead. To build a sandbox you pretty much doom yourself to running at 1/10th the speed of the underlying platform, and you doom yourself to never supporting any of the cool features that show up on one of the platforms but not the others. (I’m still waiting for someone show me a Java applet for phones that can access any the phone’s features, like the camera, the contacts list, the SMS messages, or the GPS receiver.)