Duc is a collection of tools for indexing, inspecting and visualizing disk usage. Duc maintains a database of accumulated sizes of directories of the file system, and allows you to query this database with some tools, or create fancy graphs showing you where your bytes are.
Ncdu is a disk usage analyzer with an ncurses interface. It is designed to find space hogs on a remote server where you don't have an entire graphical setup available, but it is a useful tool even on regular desktop systems. Ncdu aims to be fast, simple and easy to use, and should be able to run in any minimal POSIX-like environment with ncurses installed.