Sometimes you can't imagine how an utility can help you in your every day
work, and sometimes are the little things that makes you happy. This is exactly
the case of Folder Size for
Windows.
Windows Explorer has many lacks and Folder Size is a little addin
that permits you to solve one of this lacks: it adds a new column to the Details
view in Windows Explorer to show not only the size of files, but also the
size of folders. It keeps track of which folders you view, and scans them in the
background so you can see complete size of all files within the folder.
After the quick installation (remember that you have to open Windows Explorer
and activate it by going to Details View and right clicking on the column
headers to see a list of available columns, then choose Folder Size)
your Windows Explorer will appear like this:
This is undoubtely better that right clicking on a folder and choose
Properties...
Another little utility to add on my "must to have" list... 
Thanks Scott.