Here is a collection of gnome panel applets that I have found in the repositories, installed applets can be viewed and added to your gnome panel via right click on an empty panel space then click "Add to Panel"

Bubbling Load Monitoring
A GNOME panel applet that displays the CPU + memory load as a bubbling
liquid.
Homepage: http://www.nongnu.org/bubblemon/

Computer temperature
Computer Temperature Monitor is a little applet
for the GNOME desktop that shows the temperature
of your CPU and disks in the panel.
It also allows to log temperatures to a file and set alarms
to notify the user when a temperature is reached.
More information: http://computertemp.berlios.de/

Cpufire-Applet
A gnome panel applet showing the CPU load as a fire
A CPU load monitor, that comes as a gnome panel applet. CPU load is
displayed as a beautiful fire, the higher the flames the higher the CPU
load.

More Here