One of the guys here at work was asking for a utility that would quickly show the size of each folder on his disk.  Josh did a search and came up with Folder Size for Windows.  This utility integrates with Windows Explorer and gives you more options (columns) when viewing the file system in Details view.  New columns that you can add to your display include:

• Folder Size
• Folder Size Sort (same as Folder Size but formatted to sort properly)
• File Count
• Folder Count
• Sibling Count

Sizes are cached and updated automatically as things change on your system.  Plenty of other features to enhance performance.