Looking for a free system monitoring tool
There are two that come to mind immediately (since you mentioned iStats Menu).

1) MenuMeters is a free CPU, memory, disk, and network monitoring tools for Mac OS X. It can run in the menu bar just as iStats Menu does.



2) atMonitor offers a better alternative to Apple's own "Activity Monitor" and combines the best of the features of many other existing monitoring utilities. It also adds an extensive list of unique features of its own. In addition to the standard CPU, Disk and Network info, atMonitor allows interaction with processes, setting up triggers, logging, purging RAM and much more. atMonitor can be displayed...



As a standalone floating window...



As a menu bar...

Of the two, atMonitor is the one I prefer, but, as always, YMMV.