But another good option is Baseline, which offers both graphical and columnar views, along with a unique twist. Like GrandPerspective and WhatSize, you can scan a volume at any time and view the results. In List View, you can sort by name, kind, modification date, size, or size difference (more on the last option in a bit). Column view gives you a hierarchical view of your drive’s contents. As with WhatSize, in both of these views, file and folder sizes are colored based on size; for example, the sizes for items over 1GB in size are displayed in red, and the sizes of items over 1MB but under 1GB are purple. Baseline can even scan Time Machine volumes. (As with any of these space-surveying programs, you’ll need to run Baseline with root privileges to scan private directories; for example, the home folders of other users.)

One drawback of Baseline's Column view compared to WhatSize is that in my testing, the contents of each folder aren’t always sorted by size. On the other hand, Baseline includes Quick Look support—you can select a file or folder in any view and then press the space bar to see OS X’s Quick Look preview for that item—and you can delete or compress items right from within Baseline. (A nice safety feature: If the name of an item in List or Column view is purple, that means—in the developer’s words—the item is “referenced in Apple or non-Apple software packages that have been installed on your system.” This means you shouldn’t mess with that item.)

Like GrandPerspective, Baseline also has a TreeMap (graphical) view that shows every file on your drive represented by a proportionately-sized block or group of blocks. Click on an item to see information about it, including yellow outlines that show you the “boundaries” of the selected item’s parent folders. You also see, at the bottom of the window, the path to the item; unfortunately, longer paths can be difficult to read, as the names of each folder are cut off to allow the full path to fit the width of the window. If you double-click on an item in TreeMap view, the display changes to show a more-detailed view of the contents of the next folder down the file hierarchy.

My least-favorite part of Baseline’s TreeMap view is its grayscale presentation; I find GrandPerspective’s use of color makes it easier to differentiate between files and to visually identify groups of similar files; for example, photos or audio files. However, there’s a reason for Baseline’s approach: the program uses color to indicate changes, and the ability to display such changes is Baseline’s killer feature.

Whenever you scan a volume, the program asks if you want to save that scan as a baseline. Assuming you do so, you can later use that scan as, well, a baseline for comparison with a more-recent scan. Baseline will then tell you not only the size of every file and folder, but also how much each file and folder has changed since the previous baseline was saved. (If you’ve saved multiple baselines, you can compare the current scan to any of them.)