I stumbled across the following command while reading this article:

du --max-depth=1 --human-readable --total

This gives you an estimated indication of your file space usage by folder. Here is some sample output:

12K ./lost+found
2.5M ./bin
84K ./dev
3.3M ./etc
187M ./home
7.6M ./lib
1.0K ./mnt
1.0K ./opt
65M ./proc
65K ./root
2.3M ./sbin
3.0K ./tmp

There are plenty of other useful scripts and tidbits in the article.