Folder Size is a free tool which helps you understand how your hard drive space is being used.
Launch the program, point it at the drive or folder tree you'd like to check (C:\, C:\Program Files maybe), click Start and watch as your system is scanned (a full C:\ drive check took around 30 seconds on our test PC).
On completion Folder Size displays a list of folders in order of size. A separate pie chart presents a basic summary of the same data (you can switch this to a pie or animated chart with a click).
This list may not tell you very much, at least initially. 63.12% of our files were located in the Users folder, for instance - but which user?
To find out more, double-click a folder of interest, and Folder Size displays its contents only. This showed us which user had the most files; selecting that individual user told us their Downloads folder was a major space hog, and double-clicking Downloads displayed the ISO images which were the cause of our problems.
While Folder Size is free, the program does have Personal ($24.95) and Professional ($39.95) versions available which add extra features (they can scan network files, use command line switches, save/ load projects, filter your files, print results, export reports and more).
Slightly annoyingly, these features aren't left out or disabled. If you accidentally select one (File > Print) then you'll be told it's not available, and asked to upgrade. This can be a minor hassle at first, but you soon learn what you can do, and what you can't, and the core features are more than enough to be useful.