RIA - Rich Internet Applications.

RIA are web applications that have the features and functionality of traditional desktop applications. So users work with usual interfaces: windows, tabs, panels and so on.
To get RIA overview read article in Wikipedia. here.

There are a few frameworks to handle RIA development. My my preferable is GWT - Google Web Toolkit. This kit allows java based development with compilation to javascript. GWT supports compile time checks, good code management, testing. Ajax is also available and very easy to work with. If you have J2EE environment, GWT is ideal - it supports both client and server side development.
But GWT is good with other backends too. For example, PHP is also good source for GWT.

So if you need RIA, then GWT + J2ee or GWT + PHP is for you!

GWT is open source framework, and it has a few nice extensions useful for rapid development and styling.
GWT-EXT is on of these frameworks. This extension adds a collection of good looking elements. You may think about using part of these elements in your current web applications.

Here is two examples of collections. I have got them directly from gwt-ext samples

Sample: available elements
Sample: charts

Other good solution for RIA is Adobe's Flex. Sample for available components is here.

Other links:
RIA in Wikipedia
GWT in Wikipedia