For those not familiar with Psensor, this is a tool which can be used to monitor the motherboard and CPU temperatures, Nvidia GPUs temperature, (for ATI/AMD it's more complicated) hard disk temperature, fans rotation speed and CPU usage.

Psensor is a tool to monitor hardware temperature that integrates nicely with Unity by using an Unity Launcher counter which displays the enabled temperatures as well as an appindicator that you can use to take a quick look at all available hardware temperatures or cpu usage.