Most users will be fine with a free program such as HD Tune, which simply retrieves your hard drive’s S.M.A.R.T. data (self-monitoring information about your drive’s health) when you run it. However, if your PC is always-on it might be worth your while to keep a program like the $30 ActiveSMART (or Active SMART as it’s referred to in some areas of the program) running in the background to keep an eye on the status.
ActiveSMART is easy to install as well as easy to use. I did have its monitor crash on me a few times initially, though after reinstalling the program never wavered. Also, it’s a bit pricey for what it does–$30 at the time of this writing. If you’ve the need to monitor your hard drives on a constant basis, then it’s worth downloading the 30-day trial of this program. Otherwise, stick with one of the free alternatives.