These days we've got a lot of comments requesting for a temperature monitor applet with Ubuntu AppIndicator support that displays the CPU / Motherboard temperature, Fan speed, etc. on the panel.
Well, great news! Alex Murray has created an Sensors AppIndicator which can display various temperatures and fan speeds as well as the GPU / fan speed for Nvidia graphics cards (if you have installed the "nvidia-settings" package only!).
Indicator-Sensors doesn't have a PPA yet so I've debianized it and uploaded it to the WebUpd8 PPA so you can install it already in Ubuntu 11.04. But please note that Indicator-Sensors is currently in alpha so it may not work for everyone, or you may find bugs. If that happens, report any bugs you may find @ Launchpad.
Install Indicator-Sensors In Ubuntu 11.04 Natty Narwhal
sudo add-apt-repository ppa:nilarimogard/webupd8 sudo apt-get update sudo apt-get install indicator-sensors
Update: there's now an official PPA: https://launchpad.net/~alexmurray/+archive/indicator-sensors
Once installed, launch Indicator-Sensors from Dash (it's called "Hardware Sensors Indicator"). Then you'll need to configure it: click the appindicator and select "Preferences", then expand the various temperature modules you see in the Preferences dialog and check the box next to the temperature / voltage / fan speed you want to displayed on the panel.
Only one temperature will be displayed on the panel. To see the other temperatures you've enabled in the preferences you must click the appindicator (like you can see in my screenshot above).
To have Indicator-Sensors start automatically when you log in, launch "Startup Applications", add a new item and under command enter: "indicator-sensors".