HDD Guardian is a free open source tool which uses SMART (Self-Monitoring, Analysis and Reporting Technology) to check and display the health of compatible hard drives.

Launch the program and its Overview tab displays a basic system summary: your hard drives (SMART doesn't apply to USB keys or other devices), their temperature, reliability score, bad sectors, ATA errors and more.

The Manage tab expands on this, in particular with a "S.M.A.R.T. attributes" section which lists all the drive's raw data (spin up time, power on hours, head flying hours, it's all here).

The Manage tab also gives access to more advanced settings and options. You're able to set a spindown timer, adjust the drive Advanced Power Management setting, enable/ disable write caching and read look-ahead, run various SMART tests, even turn SMART off entirely if you can think of a reason to do so (note, most of these options need administrative rights).

Typically you'll probably just run HDD Guardian to query drives on demand, but the program can also manage and display error logs for each device. If you're particularly concerned, it's also possible to run the program in the background, and be alerted to any problems via an icon in your system tray. And an extensive Settings dialog even includes an option to receive email alerts (see Settings > Mail alerts).

Version 0.7.0:

now HDD Guardian needs .NET Framework 4.5;
changed Overview, Manage and System layout;
package now include smartctl 6.5 (issue Id #1590);
added a specific debug for devices under an ASMedia 106x SATA controller into Manage -> Advanced page (discussion #646757);
is possible to add devices that smartctl or HDD Guardian can't detect from Settings -> Device detection page (discussion #646400);
is possible to export events log also in comma-separated values format (discussion #652826);
now settings are saved inside HDD Guardian folder;
new layout for tracking and warns of disk parameters on Settings window;
now is possible to play a sound or power off PC when a device is failing or the temperature of a device is too high or in both cases;
 

Verdict

There are lots of SMART reporting tools around, but HDD Guardian provides more options and advanced features than most (and there's no annoying adware, either).