WARNING: DO NOT fill your entire drive on Linux, your system may FAIL to boot if it there is insufficient space. Keep in mind if your current user can't see a directory, these tools can't show you how much disk space those files are using.
Tools to See What is Using Disk
Disk Free simply tells you how much of each partition is used vs free
Disk Usage lists all of the files in your current directory (and all sub folders)
Command line tool for viewing which directories (and all of their contents) are taking the most space. Much easier to follow than du.
Utility to show graphically how much space each folder is taking.