Is there a widget that allows to calculate frequency for specified field of specified layer? An option to export frequency table would be great as well
May be someone have done it through geoprocessing tool?
If you have ArcGIS Server you can execute the tool in ArcMap, then publish the results as a Geoprocessing Service. You can then use the Geoprocessing widget to use the Geoprocessing service in your web application. Here is a great example of a creating a Geoprocessing service: