Linux has a few different options for monitoring your system's resources, but our favorite is definitely the lightweight, super-configurable Conky, which sits right on your desktop.
Conky is quite versatile, letting you configure every piece of data that shows up on your desktop overlay, from things like system stats to email and music. And, even though you configure everything through a text file, it's surprisingly easy to catch on. You can even download sample configuration files from the Conky web site and just load those up, without ever having to dig into them yourself