MenuMeters is a menu bar application (OS X) designed to monitor your system. It is very similar to iStat Menus but it is free and the latest version works with PowerPC and Leopard.
It can give information on a whole variety of different aspects of your system, from CPU usage to network adapters. It can provide this information in a variety of different ways (such as a pie chart). In the above picture you can see MenuMeters in use on my Power Mac G5. It shows (from right to left) CPU usage, memory usage and the Ethernet connection.
Everything is customisable. The colours can be changed, as can the width of the graph, the percentage and the type of information shown.
More information can be shown by clicking on the menu bar. For example, here you can see what is shown if you click on the CPU usage graph. Uptime, processor type and load averages are shown (plus shortcuts to Activity Monitor and Console).
Overall, I think this app is better than iStat Menus and has the ultimate advantage: it's free. Download it from here.