Baobab
Baobab is the GNOME Disk Usage Analyzer.
Baobab is a simple application which can scan either specific folders (local or remote) or volumes and give a graphical representation including each directory size or percentage in the branch. It also auto-detects any mounted/unmounted device.
Hacking
The code lives at GNOME GitLab.
Report issues to the GNOME issue tracking system.