[Osiris]

I was running the Folder Size open source program from sourceforge for some time which had a great functionality but stressed my system a lot during size calculations of all folders. This was extremely annoying when I was opening folders in explorer that were having many subfolders and files in them. It took some time and slowed down my system. I therefor uninstalled Folder Size again and decided to look out for a program that would display the folder size only if I would request the information.
I managed to find a free software that would do exactly what I needed. It is called Folder Size as well which could lead to some confusion. The biggest advantage is that it displays the size of the folders only if you run the application which can slow down the system as well but only at this time and not constantly. So, you double-click a drive or folder and Folder Size calculates the size of all folders and subfolders. You can then switch between folders to display the site of its subfolders as well.

The software is pretty easy to use. One nice feature is the so called filter list which can filter for special file types only. If you wanted for instance to only see the biggest folders that contain mp3 files or temp files you would simply use one of the many filters that are available there. You can edit the existing filters to add new file types to the filter or create new ones by editing the FolderSizeFilters textfile. Just add a new line at the end and add some filetypes to it.
Folder Size does not need to be installed, just run it from any location. The size of the application is only 300 K, works in Windows XP. Not sure about other operating systems. The developer does not mention the supported operating systems on the site.