Read about What's new in version 2.5: application builder, Chart widget, enhancements, bug fixes and much more.
The ArcGIS Viewer for Flex is a ready-to-deploy viewer application. It is configurable, so you can easily add tools and data content without programming. You can also extend its functionality with custom widgets available from the Esri community, or create custom widgets yourself with the ArcGIS API for Flex. The ArcGIS Viewer for Flex is ideal if you want more control over the look and feel of your application. It is also a good solution if you want to host your own Web mapping applications.
To download the application, follow the instructions at "Download Flex Viewer".