The Gnome 3.8 Desktop Environment supports Gnome Shell Sensors and CPU-Freq, which can be easily integrated in your Gnome Panel.
Gnome 3.10 should resolve a lot of existing problems, but since then, we have to survive with Gnome 3.8, for the moment.
The Shell Sensors and CPU-Freq extensions are quite nice and usefull.
Shell Sensors uses the lm_sensors app to monitorize the CPU temperature, so you have to install lm_sensors first, in order to add the Shell Sensors extension to your Gnome 3.8:
How to install lm_sensors on Debian, Ubuntu and Linux Mint:
$ sudo apt-get update
$ sudo apt-get install lm-sensors
How to install lm_sensors on Fedora and CentOS:
$ sudo yum update
$ sudo yum install lm_sensors
How to install cpu-freq:
How to install cpu-freq on Debian, Ubuntu and Linux Mint:
$ sudo apt-get install cpufrequtils
How to install cpu-freq on Fedora and CentOS:
$ sudo yum install-y kernel-tools