Good program to visualize file system usage on Mac?

Have a look at

• Daisy Disk

With DaisyDisk you can free up disk space by quickly finding and deleting big, useless files. The program scans any mounted disk and displays it on the sunburst map, where segments mean files and folders, proportionally to their sizes. The map is easy to read and navigate. You can also quickly preview any file and reveal it in Finder to delete.

OmniDiskSweeper is also another alternative.

OmniDiskSweeper presents you with a list of disks attached to your machine. Double-click on one, and a new window opens with a “column” view listing every folder and file you can access, which it sorts by size as you watch.

You then simply browse through the folders and files and delete the large ones which you are no longer using. If a file is part of the system, it'll say so on the panel (in the list of Packages the file belongs to), so you won't accidentally get rid of something that would make your system stop working. The free space on the disk and the ordering of the folders are automatically recalculated.

OmniDiskSweeper is freeware.

You want Disk Inventory X.

from the site:

The layout algorithm is based on KDirStat. The idea to develop this program came to me when a fellow of mine showed me his creation WinDirStat.

Disk Inventory X is freeware.

GrandPerspective is an Open Source app that will do what you're asking.