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.

• 
  

  The CPU Meter can display system load both as a total
  percentage, or broken out as user and system time. It
  can also graph user and system load and display the load
  as a "thermometer". The menu for the CPU Meter contains
  several pieces of information I like to have a single click
  away (uptime, load average, open Process Viewer, open Console).

  
  



• 
  

  The Disk Activity Meter displays disk activity to
  local disks on the system (anything that is a IOKit
  BlockStorage driver). It is hotplug aware, and will show
  activity on FireWire and USB disks as they are mounted.
  The Disk Meter menu shows volume space details for local
  drives (it does not display mounted network volumes for
  speed reasons).

  
  



• 
  

  The Memory Meter can display current memory usage as
  either a pie chart, thermometer, history graph,
  or as used/free totals. The Memory Meter menu shows a
  breakdown of current memory usage and VM statistics. The
  Memory Meter can optionally display a paging indicator light.

  
  



• 
  

  The Net Meter can display network throughput as arrows,
  bytes per second, and/or as a graph. Both the arrows and the
  graph are scaled using a user-selected scaling factor and
  calculation. Scaling can be done on the basis of actual link
  speed reported by the network interface or peak traffic and
  can use one of several scaling calculations. The Net Meter
  menu shows current interfaces and their status. Interface
  information is gathered from the SystemConfiguraton framework
  and thus is Mac OS X network location aware