Ajax.Org - Open source RIA application platform

The Ajax.Org Application Platform is a pure javascript application development framework designed to help Web developers build browser applications like traditional desktop applications. This framework provides a complete set of Web components (such as: Menus, forms, rich text editors, prompt boxes, calendars, charts, tabs, toolbars, progress bars, form validation, etc.), these Web components all have beautiful UIs (toggable skins), apply rich ajax effects, and work together to form a rich client (RIA) Web application. In the words of the official website, "Ajax.Org can fundamentally change the way you write apps" (ha ha, it's a bit loud, but it's pretty strong).