Macs Fan Control does more than its name implies. Besides providing you an option to change the BPM value of the exhaust fan, this application also gives you access to info regarding temperature sensors, details about your system, etc.
The utility sits on your Menu Bar and has a main window that shows you aforementioned system details. You can configure the app to display the current BPM value of the fan in your status bar and the temperature of any of the built-in elements (Airport Card, CPU Core 1, GPU PECI, etc.).
In my opinion, it's not the best idea to change the settings of this program if you're new to this type of applications. Altering some of its options the wrong way might cause some serious hardware problems.
However, the app comes with a small flaw: you need to restart your Mac in order to enable the logging feature (option to save the offered data).
All in all, this program provides you with a quick way to check details about your fans speed and temperature sensors, alter the RPM value of the exhaust fan, etc. Plus, it comes with a reasonable price. With that being said, it's worth buying it as long as you're an experienced Mac user.