There’s a small indicator applet that displays CPU & memory usage as percents. It’s Indicator-Sysmonitor.
Ubuntu 11.04 Unity
sudo add-apt-repository ppa:alexeftimie/ppa sudo apt-get update sudo apt-get install indicator-sysmonitor
Ubuntu 10.04 and 10.10 (32 & 64 bits)
Install the dependency
sudo apt-get install python-psutil
Download the .deb file here and install.
Now you can Open it from Applications > Accessories menu