Like I said, there are a whole bunch of options. Without knowing your actual requirements, the skillsets of the developers involved, or much of anything, it's impossible to give any useful advice. There's simple JavaScript-based solutions (jQuery, Prototype, MooTools, etc.), more "framework"-y solutions (Dojo, ExtJS, YUI), more Java-oriented solutions (GWT,
JSF), full-on client environments (Flex/Flash, JavaFX, Silverlight,
applets), and so on.