TemperatureMonitor is a free app that lets you check your system's temperature. It displays temperature values for your CPU, hard drive, battery, and several other components inside your Mac. This app is a free (or lite) version of Hardware Monitor, a more complete suite. By default, it runs both on your menubar and the system dock. But you can change this to either the dock only or the menubar only. When the app is running, you can also access a "Sensor Overview" window which shows all the sensors and their temperatures.
From the Preferences window, you can customize a lot about this app. You can change the order in which the sensors are displayed in the menubar menu and even assign them colors.
One of the most interesting features this app has is that it can keep a history of the values of all the system sensors. This is great if you want to troubleshoot your Mac or to see when it gets hot.
Also, TemperatureMonitor can give you alerts when one of the sensors goes over a specified temperature and it can also speak when values increase. This can all be very customized.
In my testing, the sensors updated in real-time and everything worked well.