Filelight allows you to understand your disk usage by graphically representing your filesystem as a set of concentric, segmented rings.
It is like a pie-chart, but the segments nest, allowing you to see both which directories take up all your space, and which directories and files inside those directories are the real culprits.

Installation

This is a cool app that available in Ubuntu Software Center, and can be found under Applications -> Accessories -> Filelight after installation.

Website: http://www.methylblue.com/filelight/