I wanted a persistent display of my CPU and hard drive temperatures on my Gnome panel (I tend to work with one maximized window at a time, so I'm not into gdesklets.) The Breezy universe repository offers the xfce4-sensors-plugin, but, for whatever reason, Breezy doesn't include sensors-applet for Gnome. Here's what I did to install it.
First, get sensors working. Don't proceed without this step. Be warned that it's possible to render your machine unbootable if you configure things incorrectly.
There are Debian packages available. I didn't use them. Maybe one or the other would work in Ubuntu; I don't know. This how-to calls for installing the build-essential package, which is a bunch of stuff you only need to compile things yourself. If disk space is tight or you otherwise don't like the idea of installing that much stuff to get one little applet, don't do it.
If your hard drive supports SMART monitoring (which it probably does unless it's really old) and you want to use the sensors-applet to display its temperature, then:
sudo apt-get install build-essential
sudo apt-get install libpanel-applet2-dev
sudo apt-get install libxml-parser-perl
When it asks you if you want it to run as a server, say yes. You can
sudo apt-get install hddtemp
if you want to change it later.
sudo dpkg-reconfigure hddtemp
Then download sensors-applet-1.5.2.tar.gz from Sourceforge. (This is the current unstable version; the release version is 1.4.)
You should now be able to right-click on your panel, select 'Add to Panel', and the add to panel window will include 'Hardware Sensors Monitor' with a computer chip icon, under Accessories. Click and drag it to the panel. Right-click on it and select properties to configure it. Here's a picture of mine, on a left-hand-side vertical panel.
cd ~/Desktop # or wherever you downloaded it to
tar xvzf sensors-applet-1.5.2.tar.gz
sudo make install