MenuMeters is a free, open-source, small-footprint set of menubar tools for monitoring CPU, disk, memory, and network usage. You set up MenuMeters in a system preference pane, adding any or all of the four meters that you want to appear in your menubar. The CPU meter displays load information along with options for graphs; the disk meter show activity on local disks, with space usage on a drop-down menu; the memory meter shows RAM usage with an optional paging activity indicator; and the network meter provides multiple options for displaying and scaling activity. MenuMeters has a long, stable history and a happy user base across multiple
OS releases, but it does use an undocumented Apple API--so interaction with other apps may cause unexpected behavior under unusual circumstances. Fortunately, as a preference pane, MenuMeters is also very easy to uninstall.