ArcGIS Viewer for Flex is a client application

ArcGIS Viewer for Flex is a client application that is built using the ArcGIS API for Flex that the user can quickly create GIS Web mapping applications without the user must master programming.

This application is designed to work with ArcGIS Server and ArcGIS Online Web services have been boosted all the latest capabilities offered by ArcGIS Server 10. For information Flex is an open source web application development tool made ​​by Adobe.
With ArcGIS Viewer for Flex, users can quickly create WebGIS applications including: data displays, interactive querying, Web editing, data extraction, geocoding, & printing and easily create customized view of the viewer as well as adding a logo or brand where everything is not require users to master the programming knowledge or programming skills.
Mapping content from ArcGIS Online is already available (preloaded) in this application, including worldwide street, imagery, and Topographic maps. To this custom viewer, users can simply edit the XML files in a text editor to change the look, functionality and content of his folder.
ESRI ArcGIS Viewer for Flex 2.0 (beta) can be downloaded for free on ArcGIS Viewer for Flex Resource Center Web page.