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.
Like Active@ Hard Disk Monitor, ActiveSMART installs, then runs as a service to keep tabs on your drive's health. The drive itself does the monitoring; these utilities simply query the drive and display the information the drive returns. ActiveSMART displays the information in a way that's easier for less experienced users to understand than Active @ Hard Disk Monitor and also offers a rather effective disk space analyzer that graphically portrays where and how of most data resides. One other feature I like is the history of the drive temperatures which can help you spot problems with your systems cooling.
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.