WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
ZetaWatch - Manage ZFS volumes in the menu bar
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.
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.
Filelight - Filelight is an application to visualize the disk usage on your computer.