i.Disk is described as 'scans directory trees and displays the total space used by each directory in an easy to use form' and is a Disk Usage Analyzer in the System & Hardware category. There are more than 50 alternatives to i.Disk for a variety of platforms, including Windows, Mac, Linux, BSD and Android. The best alternative is WinDirStat, which is both free and Open Source. Other great apps like i.Disk are WizTree (Free Personal), TreeSize (Paid), ncdu (Free, Open Source) and Baobab Disk Usage Analyzer (Free, Open Source).
WizTree scans your entire hard drive and shows you which files and folders are using the most disk space. It does this VERY QUICKLY. In fact, we believe WizTree to be the fastest application of this type currently available!
TreeSize Professional tells you where precious disk space has gone to. It can be started from the context menu of a local folder or a local drive and displays the size of each folder, including its subfolders.
Baobab Disk Usage Analyzer is a graphical application to analyse disk usage in any Gnome environment. Disk Usage Analyzer can easily scan device volumes or a specific user-requested directory branch (local or remote).
DiskUsage provides a way to find out which files and directories on the SD card of your Android phone are consuming the most space. It displays diagram with directories/subdirectories proportional to their size. Users are allowed to zoom in to look at specific directory content.
With DaisyDisk you can free up disk space by quickly finding and deleting big, useless files. The program scans any mounted disk and displays it on the sunburst map, where segments mean files and folders, proportionally to their sizes. The map is easy to read and navigate.
GrandPerspective is a small utility application for Mac that graphically shows the disk usage within a file system. It can help you to manage your disk, as you can easily spot which files and folders take up the most space.