GSmartControl lets you monitor your hard drive's health in detail
S.M.A.R.T is an acronym that stands for Self-Monitoring, Analysis, and Reporting Technology. It's a nice bit of intelligence that hard drive makers have been including into drive firmware for several years now. In simple terms, it's supposed to let you know that a drive is about to crash before the crash happens. You can then take measures to protect your data or, perhaps, return the drive to the manufacturer (if it's still under warranty).
One interesting thing is that, while drives have been making this information available for quite a while, I am not aware of any built-in Windows features that take it into account. In other words, I don't think I get an automatic pop-up saying "your hard drive is dying" when doom is impending.
Instead, to access that information and try to learn something, I need to use a standalone monitoring application. GSmartControl is one such application, and it seems very solid. It's open-source, free, and cross-platform (GTK-based, so it runs on Windows, Mac, and Linux).
It's aimed at power users and administrators. You won't find a large, green, anti-virus style button that says "Everything is Okay." Instead, you'll find a wealth of statistics with cryptic names like "Reallocation Event Count." When you hover over each, you'll get an extensive explanation of what it means.
GSmartControl also lets you test the drive. The shortest test takes two minutes and consists of a "collection of test routines that have the highest chance of detecting drive problems."
This isn't the only tool for doing this kind of thing, and it is utterly lacking in any type of visual flair. But it's free, solid, and open-source.