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 Mac 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.
What's new in version 1.8
Mac OS X 10.10 Yosemite support
ReleaseOctober 20, 2014
Date AddedOctober 20, 2014
Operating SystemsMac OS X 10.9, Mac OS X 10.6, Mac OS X 10.10, Mac OS X 10.5, Mac OS X 10.8, Mac OS X 10.7, Macintosh, Mac OS X 10.4
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for MacOS 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.
The MenuMeters monitors are true SystemUIServer plugins (also known as Menu Extras). This means they can be reordered using command-drag and remember their positions in the menubar across logins and restarts. MenuMeters is open source freeware released under the GNU General Public License (GPL).