Coretemp is really great, small footprint, cpu temp monitoring. Also, hardware monitor can and has been known to cause stability issues with certain hardware and graphics cards, namely nvidia cards, while running rivatuner for GPU onscreen display, due to over polling of the sensors.
I've had personal experiance with it, while trying to get CPU temp to show in the onscreen display overlay alogn with gpu temp etc... the only way to get cpu temp in there was to run hardware monitor, and it would cause my graphics drivers to crash, a lot.