Fortunately the developer of Baobab asked to be on bugzilla.gnome.org. Baobab is a app I always wanted to write/have, but never bothered to actually make. It shows you the used space in each directory. This is like ‘du’, but so much better. Integrates with Nautilus (you can open Baobab for a folder from Nautilus and Nautilus from Baobab).
The screenshot shows my home directory. Apparently I have 529MB of bug-buddy mails, in size that is 19.4% of all my mail. You can probably figure out that my Download folder is located under Desktop