A good disk usage analyzer is essential on any operating system. It is useful to figure out why a certain directory is taking a lot of space or to find out why you are running out of space on a partition.
I used to use TreeSize Free on Windows, which uses a treemap visualization for this purpose. Disk Usage Analyzer is a similar tool on Ubuntu. It can show the usage using either a ring chart, which is like a pie chart or a treemap. Note that the treemap visualization used here is not as intuitive as that in TreeSize Free.
Disk Usage Analyzer can be installed using the package name baobab. It can be started from the shell as baobab or from the Launcher as Disk Usage Analyzer.
Tried with: Disk Usage Analyzer 3.4.1 and Ubuntu 12.04 LTS