A modern and faster alternative to Disk Inventory X
Starting with macOS Sierra, macOS comes with an built in app from Apple called Storage Management, which is a part of the System Information app. In different tabs it shows you the largest apps installed, as well as a folder viewer with their sizes listed. A special tab shows the largest files over all of your folders, which I find very practical.
To launch Storage Management open Spotlight Search by hitting ⌘ Command  Space, type &quot;Storage Management&quot; and hit ⏎ Return.
Pros:

Free,
Fast,
Provided by Apple, so it knows what it's doing.

You can find more information here.