Conky is a free software system monitor.
It is available for Linux. Conky is highly configurable and is able to monitor many system variables including the status of the CPU, memory, swap space, disk storage, temperatures, processes, network interfaces, battery power, system messages, e-mail inboxes, Linux updates, many popular music players, and much more. Unlike system monitors that use high-level widget toolkits to render their information, Conky is drawn directly in an X window. This allows it to consume relatively fewer system resources when configured similarly.
Besides from being informative the conky can also be very eye appealing and make your desktop look even better.
Some examples from the Aureola conky collection