You like to get a clear view of your filesystem to find the files and folders that take up all the space on the hard drive? Then WinDirStat is the right tool for you, it is a free (OpenSource) program that has been originally developed under the name KDirStat for the Linux KDE Desktop.
WinDirStat is available as Windows Installer and as portable App. Both versions are compatible with the current Windows versions, I've used the application on Windows 7 and Windows 8. If you don't plan to use the application regularly or want to put it on a USB stick, then I recommend to use the portable app version which makes it easy to remove WinDirStat by simply deleting its folder.
WinDirStat can be downloaded from the vendor homepage here: http://windirstat.info/
WinDirStat installer download
WinDirStat Portable App download
I was not able to find the portable app version on the vendor page, the download link that I will provide here is from the well know computer magazine C'T from Germany: