Conky is a free, light-weight system monitor for X, that displays any information on your desktop.

This is a nice tool for monitoring your system usage.  You can customize it to fit your current theme and display just about any information you want to view.  This application really helps to keep a minimalistic desktop.  In my case, I display the time and date using conky. Doing this allows me to remove the clock applet from the Gnome panel.  I also display my disk usage, top processes, network activity, RAM usage and uptime.

This tool is definitely not a necessity, but it looks nice on the desktop and gives me up to date information on my system without having to open anything.  Try Conky out for free.