There is a free app in the Mac App Store called FreeSpace Tab (iTunes page link) that is pretty slick for anyone that needs to monitor how much space is left on their system or constantly eject drives. Once installed there will be an icon in your Menu Bar, and when clicked shows everything that is mounted on your system. You can eject any of the removable volumes (external drives, DMG images that are mounted, etc…) with a single click.
With FreeSpace Tab you can also add informative text next to the icon in the menu bar. That includes showing the disk space available of the primary drive or the percentage of space available.
The only thing I wish was added to the app is an action for clicking on a volume in the menu. Right now if you click on a drive/volume it doesn’t do anything. I think it should instead open up that drive in Finder, or something along those lines.
FreeSpace Tab iTunes Page (Mac only; Freeware)