JDiskReport
Free
JDiskReport enables you to understand how much space the files and directories consume on your disk drives, and it helps you find obsolete files and folders.
ncdu
Open Source
ncdu (NCurses Disk Usage) is a disk utility for Unix systems.
DaisyDisk
Commercial
DaisyDisk is a disk space analyzer for OS X.
Compare GrandPerspective with alternatives
Compare JDiskReport and GrandPerspective and decide which is most suitable for you.
Compare DaisyDisk and GrandPerspective and decide which is most suitable for you.
Compare ncdu and GrandPerspective and decide which is most suitable for you.