While I’ve seen SMARTReporter around for years, for some reason I never decided to use it — until now. SMARTReporter will periodically check the S.M.A.R.T. status of ATA, SATA, and eSATA drives connected to your computer (although, not firewire drives).
What is S.M.A.R.T.? According to Wikipedia, S.M.A.R.T. “is a monitoring system for computer hard disks to detect and report on various indicators of reliability, in the hope of anticipating failures.” More info here.
SMARTReporter lives in your menu bar (it’s the green hard drive looking icon on the left):

Of course, green means good. If SMARTReporter ever finds an error the icon will turn red. In the event of a failure, you can decide to have it popup an alert, execute an application, or send an e-mail (to yourself or your computer tech).
There are very few options (a GOOD thing!) with SMARTReporter. Besides changing menu bar icon appearance, and deciding what happens after a failure is detected, you can also see all SMART enabled hard drives and change the polling-interval times:

Bottom line: No Mac user should be without this super-small (~2.5MB) super-valuable tool that will help keep your data safe by predicting hard drive failures before they happen.