So far I was always using CPU Frequency Scaling Monitor

Now I came across GFreqlet
It’s a GNOME applet for Linux that not only monitors CPU
frequency scaling, but also allows the end user to change the frequency
or governor with just a click. It automatically detects which
frequencies and governors your processor supports, so there is no
configuration required.

I think it’s basically the same, but prefer now this one as it’s newer and has its own homepage.
At the moment I’m using both and the time, but I think I’m going to stick with GFreqlet even though the icon is to large for my taste.

You can get it from here