Indicator for monitoring temperature on Ubuntu

A good indicator for monitoring temperature, fan speeds and voltage is psensor. It shows output of all sensors, draws graphs. Also selected outputs can be placed in indicator panel.

It can be installed from Ubuntu repositories by

sudo apt-get install psensor

Newer versions of psensor can be installed from ppa:

sudo add-apt-repository ppa:jfi/ppa
sudo apt-get update
sudo apt-get install psensor

Here is some information with more pictures.
Another useful link
In some cases not all sensors are displayed. Then you can run

sudo sensors-detect