http://www.ragingmenace.com/software/menumeters/

"MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking for. Most were windows that sat in a corner or on the desktop, which are inevitably obscured by document windows on a PowerBook's small screen. Those monitors which used the menubar mostly used the NSStatusItem API, which has the annoying tendency to totally reorder my menubar on every login."