I have found a little tiny utility that simply and neatly displays the CPU and hard disk temperatures: the Computer Temperature Monitor (ex-Laptop Temperature Monitor)
This is a Gnome Panel applet written in Python. There are packages for various Ubuntu releases and other Linux distribution.
Tips & hints
After installing the package, it worked out of the box for the CPU temperature. Regarding the hard disk temperature, one has to install the package ‘hddtemp’ (it is in the Ubuntu Universe repository).
