Seriously, periodically do brew cleanup.

EDIT:

Seems like that since v2.0.0, homebrew decided that it should perform auto cleanup every 30 days, so it is rare to run into issues like below nowadays :) I'm keeping this up for historical reason (and possibly for other softwares that don't auto-cleanup themselves.)

I just found out that my Homebrew was keeping all the old installer for my upgradable Casks 😱

I ran brew cleanup to find to my horror:

_( _・ω)_<(me XD)@localhost H30 11/12 12:13:17)
> brew cleanup                                                                [~]
Removing: /usr/local/Cellar/apr/1.6.3... (60 files, 1.3MB)
Removing: /usr/local/Cellar/arpack/3.6.2... (22 files, 1.4MB)
...
Removing: /Users/(me XD)/Library/Caches/Homebrew/Cask/unity--2018.2.1f1,1a9968d9f99c.pkg... (940.3MB)
Removing: /Users/(me XD)/Library/Caches/Homebrew/Cask/unity--2018.2.6f1,c591d9a97a0b.pkg... (935.7MB)
...
Removing: /Users/(me XD)/Library/Caches/Homebrew/Cask/unity--2018.2.13f1,83fbdcd35118.pkg... (944.6MB)
Removing: /Users/(me XD)/Library/Caches/Homebrew/Cask/unity--2017.3.1f1,fc1d3344e6ea.pkg... (721.0MB)
Removing: /Users/(me XD)/Library/Caches/Homebrew/Cask/unity--2018.2.12f1,0a46ddfcfad4.pkg... (935.6MB)
Removing: /Users/(me XD)/Library/Caches/Homebrew/Cask/unity--2018.1.0f2,d4d99f31acba.pkg... (943.8MB)
...
==> This operation has freed approximately 19.9GB of disk space.

19.9GB?!?

I only have 250GB of space in total; that's a big chunk of data. If you are using Homebrew Cask to install your favorite apps, be sure to do brew cleanup (or use flag --cleanup on upgrades!)