i7z is a wonderful tool if you are using a Intel Core i3, i5 or i7 CPU. It shows a whole bunch of information about each physical (not hyper-threaded) core of the CPU: its actual frequency, C-state and temperature. The frequency and C-state info is especially useful since these modern Intel CPUs use Speed Step, Turbo Boost and a lot of other technologies to get the maximum performance and minimum power consumption.
$ sudo apt install i7z
$ sudo i7z
Ctrl + C to quit its interface.
Tried with: i7z 0.27.2 and Ubuntu 15.10