Conky is one of my favorite applications for all of my Linux distros. It is a light-weight system monitor (according to the project page) that can monitor many different aspects of your computer.
You choose what to monitor and you choose where the monitor is displayed on your desktop through use of a configuration file - .conkyrc. I like to have my display on the top right of the screen and I have the background transparent so it looks like it floats on the desktop.
Here is a screenshot: