System monitors are dime a dozen in all operating systems. But iStat Menus is a cut apart. In breadth and polish, it is miles ahead of every system monitor out there in every platform. And what's more: It is free (Donations encouraged). iStat tracks
After installation it can be configured using the preference panel. (System Preferences -> iStat Menus). You can choose the modules you want to enable, menubar placement and customize the looks. For example the CPU usage can be shown as a bar graph, a pie graph or as simple percentage. Every module icon has drop downs which make available a wealth of information.
One of the best modules of iStat is Calendar and Clock. I had evaluated several replacements for the default mac menubar clock but none of them were satisfactory. iStat calendar is beautiful and drool-worthy and adjacent months are easily accessible. Those looking for iCal integration will be disappointed as it is not supported but for others it is a perfect replacement for the default OS X clock. For those who need to interact with people across time zones, the world clocks feature is a life saver. iStat comes with an inbuilt database of thousands of cities and I have been able to find all the cities I was interested in. If a city is not present in the database, it is quite easy to add a custom one.
Some system monitors are resource hungry since they poll the operating system for data constantly. iStat has sensible default polling intervals which does not strain the operating system and the polling intervals for each of the modules is customizable. I recommend setting them at the maximum. Often we need to know if some process has run amok but there is no need to know it right away. A 5 or 10 second delay hardly matters. Also constantly updating statistics are a distraction.
Hovering over some items in the dropdowns opens up actions that can be performed on them. For example, hovering over a removable media gives us three 4 actions - Open in Finder, Monitor Space on Disk, Eject and Force Eject. Similarly the location field of the network dropdown allows the user to switch locations. If you have a need for using multiple locations, this is an enormous time saver.
iStat Menus is a lean, beautiful and powerful systems monitor and is a must have for every OS X user.