For Windows, I use SpeedFan to display my CPU/graphics card temperature.
Since I use a Gnome based LinuxMint distro, there's a neat little desktop applet called... Computer Temperature Monitor.
All you gotta do is open up a terminal and type:
sudo apt-get install computertemp
Once that's done, right click the tray panel and add a new widget.
Select "Computer Temperature Monitor" and it'll work straight away.
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