As I am doing lots of experiments on my SQL Server test box, I sometime gets too many files in SQL Server data installation folder – the place where I have all the .mdf and .ldf files are stored. I often go to that folder and clean up all unnecessary files I have left there taking up my hard drive space. I run following query to find out which .mdf and .ldf files are used and delete all other files. If your SQL Server is up and running OS will not let you delete .mdf and .ldf files any way giving you error that file already in use. This list also helps sometime to do documentation of which files are in being used by which database.
SELECT name, physical_name AS current_file_location
Following is the output of files used by my SQL Server instance.
Reference : Pinal Dave (http://blog.SQLAuthority.com)