Broom

Broom is a lightweight, easy-to-use disk cleaning utility for your Mac.

=lifehacker.com - "Broom Monitors Your Disk Space and Helps You Tidy Up When You’re Running Low"
=New York Post - "Avoiding that overstuffed feeling"

Broom has two life-saving features: Broom alerts you when your disk space is running low and helps you to quickly pick and choose what needs to be cleaned up.

Once installed in Menubar mode, Broom sits quietly in your menubar. One click on the Broom icon shows you how much space you've got left on your hard drive. If you're running low, Broom will let you know and help you to zero in on the biggest offenders immediately. Broom also has a standalone mode if, you wouldn't like to run it in the background, just occasionally.

Broom comes with a tab called Places to show you a quick overview of the most common places on your disk, where you can immediately free up some precious space.