GNOME Shell System Monitor is a highly configurable system monitor extension for GNOME Shell (do not confuse this with the official System Monitor extension!). It can display the CPU, RAM, Swap, Network or disk usage on the top GNOME Shell bar, either as a graph or digits (or both) and you can customize the colors, position and more.
To make it easier to install, I've uploaded GNOME Shell System Monitor to the WebUpd8 GNOME 3 PPA for Ubuntu 11.10 Oneiric Ocelot. Install it using the following commands:
sudo add-apt-repository ppa:webupd8team/gnome3 sudo apt-get update sudo apt-get install gnome-shell-system-monitor
Once installed, restart GNOME Shell (ALT + F2 and enter "r" or log out and log back in), open GNOME Tweak Tool and activate the extension.
If you're not using Ubuntu, get GNOME Shell System Monitor via GitHub.