The menu bar item
Free space at a glance, with the biggest things worth a look.
Turn it on in Cachet › Settings… with Show Cachet in the menu bar. It's off until you do.
Click it to see free space, a button to review what was found, and up to three things worth a look. With the menu bar item on, Cachet keeps running after you close its window; choose Window › Cachet (⌘0) to bring the window back.