I ran out of disk space on an ubuntu server. How can I know how the disk space is being used so I can clean up the disk / delete unnecessary files?
Try NCurses Disk Usage. It is pretty darn usable as an end-user tool (as opposed to something a programmer might use in a script).
a graphical program that I like is
KDirStat. Someone else mentioned
ncdu and it's nice.
KDirStat gives you one huge overview of your entire drive.