I have installed the
sensors-applet using the terminal line:
sudo apt-get install sensors-applet.
but there is no applet in the list of applets when I right-click on the gnome-panel.
I am using Ubuntu Natty on a MacBook Pro.
Yes, Psensor is available on the Ubuntu repositories since Oneiric.
For other Ubuntu versions, you can install it using a PPA. You will find the installation instruction in the Ubuntu dedicated page of the Psensor website