OpenRPGUI (short for Open Source RPG User Interface) is a free**, open source and light weight framework for RPG developers on the IBM i to use for building modern looking applications. We have evaluated the current infrastructure of programming in RPG on IBM i and found that the only thing we significantly lack (click here for article) is an efficient means to communicate with a modern user interface.
Our current implementation of OpenRPGUI uses ExtJS as the client framework. In the future we hope to also look into using jQuery and possibly Adobe Flex.
A good place to start with OpenRPGUI is to read the published articles that are documented on the articles page.
How many people are using OpenRPGUI? . . . check the download stats here.
** The complete source for OpenRPGUI costs no money and is made available under the LGPL license. OpenRPGUI currently communicates with ExtJS and Android devices for the UI. ExtJS is has a dual license approach that you can read about here.