Rich Internet Applications (RIAs) are the future of web applications. Unlike first generation SaaS applications that were built on a static web paradigm, RIAs bring the power of desktop-like experience to the cloud. The next generation of cloud applications is going to be RIA based, incorporating the interactivity and responsiveness you expect from a desktop application.
This is a quiet revolution of sorts since you don't get the resounding hype about the advance of RIAs. Adobe were early proponents of RIAs, creating awareness and offering both technology and tools to enable the creation of RIAs . Truth be known - there would be no momentum behind the move to RIA if not for Adobe.
This isn't simply your father's Ajax code and it's not just for the future - it's actually present reality, thanks to native web application frameworks like Sproutcore.
Want to build applications for iPhone/iPad? Sproutcore also offers a touch interface so it's enabled for mobile development.
A number of companies like Apple, Other Inbox, Eloqua have, or are currently building applications using Sproutcore. Apple's MobileMe application is a "tip of the iceberg" example of what you can do with Sproutcore. Unlike Flex and Silverlight, Sproutcore applications run within the browser without the aid of plug-ins which can often complicate deployment.