TreeSize

TreeSize Free can show only the size of local folders or local drives, not of network folders
Very good free applications to examine the foldersize of NETWORK drives are:
(1) WinDirStat: http://alternativeto.net/desktop/windirstat/
(2) JDiskReport: http://alternativeto.net/desktop/jdiskreport/
(3) FreeCommander: http://alternativeto.net/desktop/freecommander/
A very good commercial application to examine the foldersize of NETWORK drives is:
http://alternativeto.net/desktop/treesize-professional/
The free TreeSize Mobile for "Windows Mobile" is able to manage the disk space on your smartphone:
http://alternativeto.net/mobile/treesize-mobile/

I have loved, and continue to love, TreeSize Free for many, many years (more than 10 years and counting). Whenever I get a new computer it is one of the first things I install.

It is simple, fast and unbloated. So many disk usage analytic tools waste their efforts on fancy looking charts but don't help the user with their most essential goal - to quickly identify exactly what is using the most space on their storage device.

TreeSize Free, however, excels at this. It quickly analyzes your hard drive (for example, one of my drives had over 480,000 files using over 950G of space, TS Free analyzed the entire drive in under 20 seconds). It presents your entire drive as an interactive tree sorted by file size (or even file counts, if you prefer), allowing you to expand or collapse each directory so you can quickly zoom in on exactly what you need to remove in order to free up the most space in the shortest amount of time.

As an example, I launched TS Free on one of my computers. This one has a 1TB drive but less than 60G free space remaining. I knew it had lots of downloads and photos - but not that much! What was taking up so much space?? It took TS Free 17 seconds to analyze this drive. Right away the problem is revealed - over 460G is consumed by 18 files in "C:\System Volume Information". After a quick online search I found that these are system restore points, and the amount of space they occupy is configurable via the system protection utility in W7 (but that's a topic for another day). All I had to do was adjust the system protection options to reduce the restore point disk consumption. But the important point here is that I quickly got to this solution with excellent assistance from TreeSize Free.

The biggest limitation of the free version (AFAIK) is that it won't analyze network drives. That hasn't been a big issue for me, so I haven't had to opt for the Pro version.

[Edited by rmbjr60, April 26]

The free version is great, but check out the full version to get the whole range of features.

TreeSize is fast (even faster in new version 6), comes with a great file search feature, supports NTFS, can find and remove duplicate files and is all in all the best application for disk space management I know. For servers I use the "big brother" SpaceObServer.

[Edited by noellehager, May 19]

This great application supports junctions and do not add junction's target sizes in reports!

But it do not understand hard links, and it's sizes in report are duplicated (

[Edited by Nashev, April 22]

Non-free version supports hardlink resolving! And deduplicatioin by creating hardlinks for same files have!

The free version does what it says. It does it quickly and easily.
Its interface is minimal and beautiful so it doesn't tire the eye.