This is a system load monitor capable of displaying graphs for CPU, ram, harddisk, and swap space use, plus network traffic.

To install System Load Indicator, Ubuntu 11.04 and 11.10 user run following command in a terminal window to add the ppa:indicator-multiload/stable-daily

sudo add-apt-repository ppa:indicator-multiload/stable-daily
sudo apt-get update
sudo apt-get install indicator-multiload