Mister Plimsoll is a MacOS app by Glenn Fleishman that does one thing very well: it shows how full your disks are and warns you when they're getting full.
Mister Plimsoll is a small menu bar utility for macOS that keeps tabs on whether you are running out of storage on the volumes you choose to monitor. You can get an alert, notification, email, iMessage, or Pushover whenever any one of them gets too full. Set a threshold for each drive — internal, external, Time Machine — and Mister Plimsoll will alert you before you run out of room.
You can set per-volume warnings, email alerts, and hook it up to Slack or anything else that uses Webhooks.
The name is the best thing about it: "The Plimsoll mark is the load line painted on a ship's hull — the limit past which you can't safely add more weight without risking the boat. It's named for Samuel Plimsoll, the British MP whose 1876 Merchant Shipping Act made it law. The mark is the universal reminder that there is such a thing as too full."
On Linux, there's Disk Usage Analyzer and Filelight; you don't get alerts. There doesn't seem to be anything quite so tastily minimal on Windows, but I've never really looked because the maximal WinDirStat is always one of the first things I set up there. If you want the visualization on Mac, GrandPerspective has that.
