MenuMeters is a set of CPU, memory, disk, and network monitoring tools for MacOS X. The MenuMeters monitors are true SystemUIServer plugins (also known as Menu Extras). This means they can be displayed on the menubar and reordered using command-drag and remember their positions in the menubar across logins and restarts.
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 or as used/free totals. The Memory Meter menu shows a breakdown of current memory usage and VM statistics.
The Net Meter can display network throughput as arrows, bytes per second, and/or as a graph. The Net Meter menu shows current interfaces and their status. Interface information is gathered from the SystemConfiguraton framework and thus is MacOS X network location aware (to prevent interfaces from appearing in this menu see the FAQ in the ReadMe file included in the download).
More info and download link here.