Mac OS X only: Freeware application SpaceControl keeps watch of your hard drive space and alerts you when you’re getting low on free space. The application lives in your Mac’s menu bar and displays the total amount of free space available on all drives. You can also set alerts to notify you with a simple system sound or with an email when your startup disk drops below a threshold you define. In general iStat Menus is an excellent menu bar app for monitoring you system (including hard drives), but if you have trouble keeping enough free space on your drive, SpaceControl might be for you. SpaceControl is freeware, Mac OS X only.
