This little utility is what helped me get back to folding. I was using AMD OverDrive to lower the CPU voltage and so make the CPU cooler while folding, but I often put my PC to sleep and when it wakes up the voltage is up to the usual, and it drove me nuts. I tried voltage control in the BIOS but it seemed to have no effect except making my PC less stable. Then I found this utility (thanks to someone at Anandtech) and it not only helped me fix the voltage but also explained to me why the BIOS and AOD differed.

PhenomMsrTweaker can set the clock multiplier and voltage for each performance state. After looking at it I understood that the BIOS was only controlling P0, which is the turbo state on the Phenom II X6, while AOD was controlling P1, which is what's running when all cores are enabled. I'm now running at 1.225V instead of 1.35V, which lowers the core temperatures by 6-7 degrees C.

I imagine that AMD CPU's without turbo can be easily controlled through the BIOS, unlike the Phenom II X6, so such a utility is less useful for these CPU's, although it probably still provides more control and does it easier than the BIOS can. It's the simplest way to control clock and voltage that I know.