I fixed this issue by doing two things.
First thing (saved 18.5 GB):
Context
I used to have multiple users on the machine, so I kept lots of stuff in the "Shared" folder (iTunes Library, iPhoto Library, iMovie Library, etc.). It turns out that this folder gets classified as "other." Also, it turns out that that libraries can take up more space than needed if they are in that folder.
Steps I took
- Went down to one user
- Moved libraries in shared folder into user folder (iTunes library was 33GB in shared folder...it was only 29.95GB in my user's "Music" folder, similar shrinking phenomena with other libraries).
- (After libraries were in user folder) I deleted the libraries from the shared folder (since moving them only really "copies" them into new location).
- Empty trash
- Forced Spotlight to re-index.
- Reboot into recovery mode (restart, hold command+R after boot chime).
- (from recovery mode) Choose Disk Utility
- (from disk utility) Click my harddrive ("HD" top left)
- (from disk utility) Choose Repair permissions
- (from disk utility) Choose Repair Disk
- Restart (from Apple logo, top left).
Without deleting any files (just moving them repairing hard-drive), this saved 18.5GB of data total. No idea why.
Second Thing (saved 25GB)
I used Omnidisksweeper to find abnormally large files in my "Users" folder (NOT "usr" folder!). I found a handful of folders that belonged to apps that had deleted ages ago (e.g., PDF-Compress). Apparently, they stored stuff in the hidden "libraries" folder without notifying the user. Deleting these files saved another 25GB.
Pondini's Disclaimer:
Do NOT mess with anything you don't normally see, unless you fully understand it, or as noted here -- such items are hidden for a very good reason! They'recritical system stuff. Deleting the wrong thing can make your system unusable!