From Jude Giampaolo:
Drive Monitor is a free utility for the Macintosh that allows you to easily view the status of all of the volumes mounted on your desktop. The main window shows the name and icon of each volume as well as their size, used space, free space, percent used, percent free and a "thermometer" graphic representation. It is possible to sort the display by any of these items. If a volume is locked, a padlock icon is shown to the left of the drive name. Nearly full volumes are hilighted in a used selectable color. The bottom of the window provides a summary of all of the drives shown.