Adobe FLEX... I made a project using it
its actually a RIA (Rich Internet Application) implementation framework from Adobe (similar are Silverlight from Microsoft, etc)
its SDK (command-line) is Open-Source
its IDE (too good, built on Eclipse) is available free as Trial
main code for GUI is in MXML (which is too easy, just a variant of XML)
like for a button
Code:
<mxml:Button id='btn1' click='axn1();'/>
now these PROGRAM LOGIC functions can be written in Actionscript 3.0 as an Inline or External Script, as inline between tags
Code:
<mxml:script><[CDATA[
//code
]]>
</mxml:script>
for Server Side Coding... you need to use their Data Service Gateway (LifeCycleDS is proprietary or BlazeDS is OpenSource)
using it the Server Logic can be coded in any of ASP.net, PHP, J2EE or ColdFusion
...Writing a Mini Tutorial on Flex... soon gonna Post a link here too... [

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