Conky is a system monitor. It handles a bunch of different system variables, including CPU, disk space, running processes, RAM, temperature, network transfer, and log data. Conky is a fork of torsmo. You can customize the layout, colors, and fonts. It can be a bit of a bear to configure --- it requires significant work with the .conkyrc file in order to get it up and running.
Fortunately, you can find a variety of good configuration templates and files all over the internet. (Hat Tip to lyrae for the weather script I currently use). Installing the package takes the normal route. Fire up a Terminal, and type the following: