We use a tool called FolderSizes to manage disk space. It provides not only a hierarchical treemap view like WinDirStat, but also many other useful reports and tools. Note that it is NOT freeware, but the cost is very affordable and quickly pays for itself.
FolderSizes has the ability to report on actual and allocated disk space, and also shows oldest, largest, temp and duplicate files. It also classifies files by size, name length, owner, type, and more. Everything can be reported, exported, scheduled, etc.
But my favorite tool is the built-in metadata search. You can formulate complex file system queries to answer questions like "Who owns the largest files on this disk?" or "Which files haven't been modified in over a month?"