Lm-sensors

Lm-sensors let's you monitor your system's temperatures for individual components.

Installation:

 apt-get install lm-sensors

Detect sensors:

 sensors-detect

Show temperatures:

 sensors

Hard drive temperature monitoring:

 apt-get install hddtemp

Show HDD temperature:

 hddtemp /dev/hd?

/dev/hd = the hard drive you wish to measure