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]

External resources

 * Conky homepage
 * Conky FAQ
 * #conky IRC chat channel on Freenode
 * Conky page on ArchWiki