Acronis Drive Monitor is a free tool that will keep a close eye on your hard drives, and alert you to problems that could indicate an imminent failure.
The program works with S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) compatible drives, monitoring their spin-up time, temperature, seek errors and more than 20 other parameters. If there's a problem then you get an immediate alert, a warning icon appears in your system tray, and double-clicking this provides more information.
The alerts are provided in a very clear and non-technical way, but if you want access to the raw S.M.A.R.T. data then that's available, too - just click the "S.M.A.R.T. parameters" tab and get an in-depth view on your drive's health.
Drive Monitor also monitors the Windows event log for critical events that may indicate worsening drive problems. These include file system corruption, failed read and write operations, Volume Shadow Copy failures, and more.
And if you have an Acronis backup tool then you'll also be warned if the last backup failed, or you've not backed up at all recently. (Of course if the program is reporting significant problems then you'd be advised to make a full backup, right away, just in case the drive is about to crash.)
Drive Monitor is worth installing, if only briefly, just as a quick check on how your hard drives are doing. On our test PC, for instance, it reported that one external drive's temperature was above the warning limit, which will shorten its life. We repositioned the drive, and within 20 minutes its temperature was within normal limits.
You shouldn't feel worried about leaving Drive Monitor running for longer, though, as it consumes very little resources, minimal CPU time and under 3MB (private working set, for the system tray application) of RAM on our test PC.
Note that the download here will point you to the Acronis page where you can register and download Drive Monitor.