Back in the hoary days of System 6, the Finder badly needed help, and DiskTop was one of my favorite helpers. Over the years, DiskTop somehow fell off my radar screen, though I was dimly aware that CE Software had spun it off to the Prairie Group; and TidBITS hadn't reviewed it since 1994, when Stephen Camidge looked at DiskTop 4.5. Having bemoaned the frequent untimely death of good software, I was stunned and delighted to learn that DiskTop is still available - and still works, though the version number has increased only to 4.5.3.
DiskTop is a single window displaying one folder's contents, like a non-hierarchical version of the Finder's List view, but including invisible files, and telling you type/creator codes and exact data/resource fork sizes. Navigation between folders (using always this single window), by mouse or keyboard, is lightning-fast; you can also nominate favorite folders for direct access through a pop-up menu. Supplementary modal dialogs let you delete or rename an item, pick a folder to copy or move an item to, create an item, learn a folder's size, or copy a pathname. A Get Info dialog lets you get (and set) the sort of technical stuff for which you'd otherwise need ResEdit or Snitch. You can also find by multiple criteria, quickly and easily.
To be sure, DiskTop has its failings. For example, it lists invisible files, but it doesn't tell you they're invisible, nor does it let you search only for invisible files. As you make an alias, you can't dictate its name. And so on. But carping, though easy, is pointless, since these issues will probably never be addressed. In the past six years, DiskTop has been tweaked to ensure compatibility with Apple Menu Options and Y2K, but functionality remains unchanged. Indeed, much of DiskTop's appeal, I have to admit, is that it's such a blast from the past. It's tiny (220K, using 80K of RAM). It's a desk accessory (remember those?). It comes on a floppy! It's not PowerPC-native. It doesn't use drag & drop. It opens files and folders, not through the scriptable Finder or other modern methods, but through the antiquated CE Toolbox extension (this rather hampered my system, and ultimately I elected to forego this functionality). The main downside is that it costs $50, which seems rather cheeky for software that isn't being updated; does Prairie Group think software improves by sitting, like wine?
DiskTracker -- For a thoroughly modern alternative that's being updated regularly, you might try the $30 shareware DiskTracker, by Mark Pirri (Portents LLC). This, too, is a blast from the past, but in a different way: it goes back only to 1996, but its conceptual ancestry reaches well into the 1980s to another old favorite of mine, Bill Patterson's FileList+ (itself based on Erny Tontlinger's FileList).
DiskTracker was originally a file cataloger, meaning that it quickly reads and stores into a single document the file information from as many disks as you like. Catalog file size is roughly proportional to the number of files; the catalog listing all 27,000 files on my hard disk occupies 1.5 MB of disk space.
But with the recently released DiskTracker 2.0, if the disk whose catalog you're viewing is mounted and writable, you can make changes to it through the catalog. As with DiskTop, files are listed in a single window, and you can navigate into a folder using the same window; but you can also view folders hierarchically, as with the Finder's List view. You can rename items, delete items, move items to the Trash, copy an item's path, create a new folder, and view and alter an item's type/creator, creation/modification date, and locked and invisible attributes. Copying files works through drag & drop, which is clumsier than DiskTop's dialogs because of the single-window approach - what I'd really prefer is a Windows-like cut-and-paste metaphor - but it operates both internally and with the Finder, which is slick. You can open or reveal the actual item in the Finder. You can't create an alias; resource and data fork sizes aren't listed separately; invisible items can be shown or hidden, but there's no direct indication that they're invisible. On the whole, it's like a modern DiskTop.
Additionally, you get DiskTracker's disk cataloging features. In particular, you can do highly complex saveable searches, which result in a flat sortable list of the matching items. For example, show all your files in a flat view; then sort them by size to learn where your hard disk space went. Search for duplicates based on criteria that you specify (I instantly found 14 MB of unneeded files). Plus, don't forget, you can search disks that aren't mounted. Oddly, unlike the regular view, the flat list view can't be customized to display extra columns such as type/creator, nor to show creation/modification times along with dates, nor can columns be widened or moved.
DiskTracker is a 1.3 MB download, and although its requirements aren't as minimal as DiskTop's, it requires only a 68020 or later Mac with System 7 or later and 2,000K of free RAM.