As you can probably tell from its name, Drive Alert is a useful Mac application that was designed to warn you of any possible hard drive failures. With the help of this tool, you can determine if it is the time to create backups of your data so that you won't risk losing it in case of a hard drive failure.
The warning system that the Drive Alert puts at your disposal is very simple. If, after the verification, your hard drive is in a good state, the icon from the Finder menu appears green and, if you see the icon red, then it is time to take the appropriate measures like creating backups of your important files or taking the hard drive to your local Apple store.
Even though the Drive Alert doesn't come with an actual graphical interface, you can access the "Preferences" menu where you can modify the drive you want to monitor (only supports ATA, SATA or eSATA internal drives) as well as the interval in which your selected drives are verified (from 15 minute checks to daily verifications).
Other than that, not much more can be said about the Drive Alert, besides the fact that it provides you with activity logs with the help of which you can determine the exact time when your hard drive began to deteriorate.
To sum it all up, if you are ever in need of an easy-to-use Mac application that will warn you about impending hard drive failures, then the Drive Alert is a good choice.