My favourite tool to answer this question and give insight into what is taking up hard drive space has been WinDirStat for a number of years now.
Based upon the discontinued KDirStat for KDE3 it scans the entire drive and presents the findings in a number of ways.
- A directory list in a tree view which you can sort by drive usage;
- an extension list shows different file types and statistics;
- and a very pretty treemap!

It just works and does exactly what it needs to!
What’s your favourite tool to answer this question?