There's a rough open source clone of Filelight for Macs[1]. Disclaimer: I'm the original author, but no longer involved.

A commercial app in the same genre, DaisyDisk[2], is much more flashy and polished. You can also get the KDE/Mac version filelight from MacPorts, as part of package 'kdeutils4'.

[1] https://github.com/jvhaarst/MacFileLight [2] http://www.daisydiskapp.com/