Question: I'm interested if there is any ArcGIS tool which give us the same result like GRASS command r.sun. I need to get drop shadows. You can suggest some other software?

Answer: 
You can use **Spatial Analyst** extension from ESRI to perform the solar radiation modelling.  

Refer the link below:  

<http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=An_overview_of_the_Solar_Radiation_tools> 

1. Area solar radiation analysis
2. Point solar radiation analysis

Cheers!
The equivalent tool in ArcGIS is the [solar radiation toolset](http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/An_overview_of_the_Solar_Radiation_tools/009z000000t4000000/), part of spatial analyst. I'm not sure of exactly what you mean by 'drop shadows', but it does support control over a large number of model options.