A while ago a disk of one of our DigitalOcean droplets had almost no free space. Ncdu (NCurses Disk Usage) is a command line tool to view and analyse disk space usage on Ubuntu. It can be installed with this command:
sudo apt-get install ncdu
Once it has been installed it can be launched by simply typing
ncdu in the terminal. After it has done it's initial indexing you'll see a screen not unlike this one:
The arrow and enter keys can be used to navigate the file listing.
Using ncdu I quickly found a few big directories and could free up some space.
Bonus tip: when my mac complains about a full disk, GrandPerspective is my weapon of choice to hunt down big files and directories.