I started with Flex and the Sample Flex Viewer many years ago and have been building widgets and customizations for the viewer ever since. Back when it was called the Sample Flex Viewer (SFV) there was a small community of developer and adopters that took the SFV and made it their own, customizing the SFV source code and rewriting how the SFV core code did things and extended its functionality. Today now that the ArcGIS Viewer for Flex is a fully supported product from esri developers like myself are less likely to change the viewer source code and have focused on adding additional functionality only using widgets. I have taken many esri out of the box (OTB or OOTB) widgets and enhanced them with more and better functionality.
Here is a listing of those widgets in order of popularity: