ncdu

From the man pages:

ncdu (NCurses Disk Usage) is an interactive curses-based version of the well-known du(1), and provides a fast way to see what directories are using your disk space.

Silly simple, yet useful. Fun application: visuall browse your server’s file system:

ssh user@server "ncdu -o - /" > /tmp/server_scan.json
ncdu -f server_scan.json