Folder Size for Windows adds a new column to the Details view in Windows Explorer. The new column shows 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.

Features:

Don't switch to another program to see folder sizes. It's always there when you're viewing your folders (even in Open and Save file dialog boxes, you can change the view), and it's a great visual cue to help you find the one you're looking for. Not just for disk cleanup, but for any time you need to choose a folder.
No scanning phase before you can start. As soon as you display the Folder Size column, you'll see sizes immediately. Large folders will continue to scan in the background while you browse your folders.
Explorer will automatically update the folder size column in real-time. No more manual rescanning.
Background scans won't monopolize your disk. When Folder Size detects that other programs are doing a lot of reading to and writing from the hard disk being scanned, the background scanning will wait for the other programs to finish.