WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.

Tune Sweeper - Show duplicates in your Music library. Automatically select which duplicates to keep and delete based on your preferences. Remove duplicates with one-click from your iTunes or Music app library.

WizTree - WizTree quickly finds the files and folders using the most space on your hard drive. It scans the MFT (Master File Table) instead of crawling the entire disk which makes it very fast.

Dupe Eliminator for iTunes - iOS, iPad, iPhone, iPod Touch, iTunes, iTunes software, Software for iTunes, mobile iOS apps for iPad, iPhone and iPod Touch. Remove iTunes duplicates. Stream iTunes Music Library. Super iTunes Suite of useful tools.

SpaceSniffer - SpaceSniffer is a freeWare (donations are welcome) and portable tool application that lets you understand how folders and files are structured on your disks.

iTunes Duplicate Song Manager - iTunes Duplicate Song Manager with Folder Watch removes iTunes duplicates in Windows and Mac OS X.