Like MiniUsage, SpaceControl sits in your menu bar and shows the remaining free space on your startup volume. Click the SpaceControl icon and you see a list of the remaining space on each connected volume; you also see the sum of these numbers--the total free space available on all volumes. (Unfortunately, there's no apparent rhyme or reason for the order of non-boot volumes in the menu. It would be nice if you could choose the order manually, or at least if the volumes appeared in alphabetical order.)

Displaying free space isn't an unique feature; a number of other utilities provide similar functionality, including the excellent . What makes SpaceControl especially useful is its notifications. As I , it's important to keep an adequate amount of free space on your boot volume: If your hard drive gets too full, performance can suffer and you run the risk of losing data.

SpaceControl actively watches your boot volume, and if the amount of free space dips below a configurable threshold (by default, 10 percent, but you can change the amount), you see an onscreen warning, receive an email, or both. The former option is useful for computers at which someone is actively working. The latter option is great for remote servers; it's also useful for computers you maintain, officially or otherwise--say, the ones you take care of for family members spread out in various geographic locations.

SpaceControl offers the same notification service for non-boot volumes, and, in fact, lets you set a different threshold for those. (I've got mine set to 10GB.) You can choose to monitor all non-boot volumes (which, unfortunately, also includes mounted disk images and thumb drives), or just specific ones. The downside of this feature is that you can't choose a different threshold for each non-boot volume; for example, I want to know when my Boot Camp or Media drive dips below 10GB, but I don't really care about my scratch drive (which stores temporary files and downloads) until it dips below 1GB.

I also wish there was an option to hide the remaining-space display on the menu bar--showing just the SpaceControl icon--in order to save menu-bar space. And I found that SpaceControl's text/number fields are a bit hinky: I occasionally had to close and re-open the program's Preferences window to be able to enter text and numbers. But the program's core functionality works well, and the program has already notified me several times about drives that are getting full.