Nodesoft Disk Bench - This software is a benchmarking tool that lets you know the exact real world data transfer rate while actually copying a file to or from your hard drive. You can select any file for benchmarking and know the data transfer rate and time taken to transfer. This is useful in case you need to compare the speeds of two hard drives.You can download it here.
Seagate Seatools - This application functions as a diagnostic tool. It allows you to test an internal or external drive by any manufacturer. Apart from the usual troubleshooting tests, it features an acoustic test that helps you find out how much noise your hard drive is producing.
Lavasys Everest Home Edition - This is a monitoring tool scans all the hardware in your system and you can view the status of not only your hard drive, but your CPU, fan speeds and motherboard. It displays the individual temperature of each hard drive in your system which enables you to find out if any of the hard drives are overheating due to some fault. You can download the software from here. The parent company has unfortunately stopped development on the freeware version of this application, but the last available freeware version does a great job of hard disk monitoring.
HD Tune Free Edition - This application is a benchmarking and monitoring tool. It checks your hard drive's health using SMART, scans your drive surface for errors and also displays its temperature. It also has a benchmarking feature that displays a graph which shows a graph of the transfer rate and access time. Burst and random access rates are displayed. The website of the product has a listing of benchmark results of the most popular hard drives which you can use for comparison. You can download the application here.
Scandisk - Microsofts very own Scandisk application is a basic but effective freeware diagnostic and testing tool that you should use for checking for bad sectors on your hard drive. Since it comes bundled with your OS, all you need to do is fire it up from the accessories section on your start menu.