Conky is a free, light-weight system monitor for X, that displays any information on your desktop.
Install
Open a terminal window and run:
sudo apt-get install conky -y
Usage
Conky can be accessed by typing the following command: conky. You can use the -c | --config= FILE option to load a configuration file instead of $HOME/.conkyrc.
My suggestion is to have a folder with some Conky configuration files and add Conky to session startup[1] typing conky -c path_to_conky_rc as the command. Please don't forget to adjust the configuration file path.
Where to get examples
Great examples can be found at deviantArt.com. There's also examples at gnome-look.org under the "Other Gnome Stuff" section, but it's easier to just search for "conky". After downloading a Conky configuration file you may have to edit it to make everything work.
Footnotes
[1] See this to learn how to add a program to Xfce session startup.