Folder Size for Windows adds a new column to the Windows Explorer details view that displays the sizes of files and folders. A service scans your hard disk in the background and caches the results. Designed for performance!
Does NOT add a column, but has a separate pop out window with a duplicate list of folders with sizes. BUT you cannot delete, move, copy, rename, or do ANYTHING... so it is helpful to a degree, but would be a LOT better if it was just a column added.