I was running out of space on a server the other day and was wondering what directory was full and what files I could clean up. There is a simple command that is helpful to identify usage by directory. This works from within any directory so you can walk the path from / down through your largest directories and look for files to clean up.
[root@localhost]# du -hc --max-depth=1
Thanks, good to know.
Also, if you're in X there are some GUI tools that give a nice visual pie chart of usage on your drive.
Filelight (QT app) and Baobab (GTK/Gnome app).