WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
DaisyDisk - DaisyDisk is a disk analyzer tool for OS X that visualizes hard disk usage and allows to free up hard disk space.
Disk Space Pro - Scans your home folder and reports folders and file disk space use.
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.
Speccy - Speccy - find the details of your computer's specs. Great for spotting issues or finding compatible upgrades. Download the latest version free.