For AMD processors there is the excellent cpupowerd.
there is a ppa providing it (
sudo add-apt-repository ppa:autostatic/ppa ), but compiling is painless.
make && sudo make install should do fine.
It needs the msr kernel module loaded (just add it in a new line to
Play around a little:
sudo cpupowerd -r 0.01 is a good starting point. This reduces the voltage of all cores by .01 Volts for all frequency steps.
It's best to run cpupowerd as a service when you've found the appropriate values. To do so go to
/etc/init.d copy skeleton to cpupowerd and insert the values that are stable for you. Be sure to always test your settings under high load.
In my case it saved even more power because the tiny undervolting was just enough to keep the fans turned off when I use my laptop for browsing.
Edit: For now cpupowerd supports only AMD. The README says that possibly Intel will be supported as well. The status of the project is unclear though.