AJAXanchors the lightweight end
AJAX is used to extend a wide variety of Web server application technologies. You can, of course, use it to enhance otherwise static HTML pages with data-driven content, but it's more customary to couple AJAX on the client with scripted Web servers. Ruby on Rails makes adding AJAX features especially easy. Microsoft ASP.Net AJAX, as the name implies, enhances ASP.Net sites with AJAX features, and includes Visual Studio integration; the Microsoft AJAX Library can also be used with other types of sites. Integrating AJAX libraries with Java/JSEE sites can be done manually with some effort, but tools and libraries such as the Google Web Toolkit, Tibco General Interface, and ThinkCAP JX make it much simpler.
.Net and Java on the heavyweight end
The heavyweight end of the RIA spectrum is shared by .Net Smart Client applications and Java Applets deployed over the Web. In both cases, the client computer needs to have a fairly large (more than 10MB) runtime engine installed before it can meaningfully download the RIA.