Passmark DiskCheckup is a free, simple SMART monitoring tool
It's a simple little tool which keeps an eye on the SMART status of your hard drives. In addition to displaying an exhaustive amount of information about your drives (everything from its model and serial number to standards compliance and feature support), DiskCheckup can also park itself in your system tray and keeps a watchful eye on HDD temperatures. And yes, Erez, you can choose between Celsius and Fahrenheit...
Once your threshold has been crossed, DiskCheckup can either display a notification or send you an email notification.
One tweak I'd like to see added is for DiskCheckup to display drive temperatures when hovering over the system tray icon. It's not critical, however, because I'm far more interested in being notified when something is wrong -- but it's still nice to have the option.