Conky

Conky is an advanced and highly configurable system monitor for X, which "can display arbitrary information (such as the date, CPU temperature from I2C, MPD info, and anything else you desire) to the root window in X11. Conky normally does this by drawing to the root window, however Conky can also be run in windowed mode (though this is not how conky was meant to be used)." [Conky FAQ]

Installation
Install :

After Installation / Editing your conkyrc
After installing conky, copy /etc/conky/conky.conf to ~/.conkyrc so that you can easily edit your conky config

Once you have copied this file, you can begin editing your conkyrc, to do so, use nano or any other text editor

Other resources

 * Gentoo CONKY Guide
 * Conky FAQ
 * #conky IRC chat channel on Freenode
 * Conky page on ArchWiki