WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
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.
TreeSize - TreeSize tells you where precious disk space has gone to.
Baobab - Distributor of workload automation, financial statement reporting, data access, IT Asset Management and Service Desk software solutions.
SpaceMonger - SpaceMonger helps you understand what's occupying your hard drive and more effectively clean it up without worrying about losing anything important in the process.
DaisyDisk - DaisyDisk is a disk analyzer tool for OS X that visualizes hard disk usage and allows to free up hard disk space.