DirSize

  Command line tool that can easily be automated to generate nightly reports of disk space usage, network space hogs and very large folder trees. Reports can be sorted and small folders can be prevented from being displayed. Both 32 bit and 16 bit versions are included, all support long filenames. Full scrolling support in either direction. Now includes HTML output option.

Its uses include:

Finding space hogs on a network
Finding multiple copies of a single application on a server
Finding which folders could be archived to save space (typically those that contain lots of small files)
Finding where most of the disk space is used
Finding applications to remove before installing a new one
Previewing the benefits of a smaller cluster size
Generating daily, weekly or monthly reports showing how the disk space distribution has changed. You can easily generate tab- or comma-delimited output to feed directly into your favourite spreadsheet for further analysis
Showing you the complete folder tree