RFA (Revit FAmily) is the format of families/blocks of the BIM application Autodesk Revit. Many entourage blocks from the "CAD blocks" section are available for download in the RFA file format.
There is no standalone converter for this format, but for one-time conversion of RFA to standard CAD formats you can use the Trial version of Revit Architecture. It can be downloaded from www.cadstudio.cz/download.
Open the RFA file in Revit (e.g. by simple drag-n-drop) and using the menu R > File > Export > CAD formats choose export to the format DWG, DXF or DGN or SAT. You can specify other export options and complete the conversion.
Note: make sure you switch first to the view you want to export (e.g. 3D, plan, elevation, etc.).