Temperature indicator for gnome panel

sensors-applet will do the trick for you. You can install it with:

sudo apt-get install lm-sensors sensors-applet 

You should then do sudo sensors-detect so that lm-sensors is configured (it will scan sensors in your hardware).

Then simply add "Hardware Sensors Monitor" to the panel. Then, within the context menu, you will be able to configure which sensors will be shown, units, update interval....

sudo apt-get install sensors-applet

Then when you right click the panel, and go to "Add to panel..." and add a "Hardware Sensors Monitor", you'll see something like this:

And yes, as luri points out, you'll also need to set up lm-sensors for this:

sudo apt-get install lm-sensors
sudo sensors-detect

Go through that and find out what modules need adding (you can have it write to modprobe directly in the last step) and then you should be good to go.