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For OS X, http://www.derlien.com/ is a solid alternative.

Or DaisyDisk, I love DaisyDisk.

or OmniDiskSweeper.

https://www.omnigroup.com/more/

Space Gremlin is another good one for OS X. and it feels a little more "Nativey" than Disk Inventory X (which I've also used and is also good).

http://www.spacegremlinapp.com/

GrandPerspective is also good, despite its hosting.

http://grandperspectiv.sourceforge.net/

Alternatively for windows, SpaceSniffer is a portable single file under 1 meg and no installation is required. http://www.uderzo.it/main_products/space_sniffer/

Definitely an upgrade to the old WinDirStat.

WindirStat was based on KDirStat[0] (which apparently has been superseded by qdirstat[1], this is the first I've heard of it) for KDE. Baobab[2] is a similar tool for Gnome.

[0]http://kdirstat.sourceforge.net/

[1]https://github.com/shundhammer/qdirstat

[2]http://www.marzocca.net/linux/baobab/

A bit slower, but I just use totalcmd to display sizes of all direct subfolders. Check both program files' folders and then user folder (and work my way into appdata).