Hot on the heels of my story yesterday about undervolting your Core Duo CPU to save on battery and heat comes a nifty piece of shareware. CoolBook is a GUI to control CPU clock speed and voltage. It's like the author was reading my mind!
CoolBook is a $10 shareware application for the MacBook and MBP that allows you to adjust the frequency (clock speed) and voltage of Intel Core Duo and Core 2 Duo CPUs.
According to the author's published benchmarks his MacBook (1.83GHz) temperature decreased by as much as 14°C (25.2°F) just by dropping the voltage from 1.2125 V (Apple default) to 0.95 V (the minimum).
Other programs I've convered here before allow you manually control the MacBook and MBP's fan speed which can greatly reduce operating temperatures.
In my experience Core 2 Duo MacBook Pros can run as much as 40°F cooler than Core Duo machines. I don't consider these voltage/frequency/fan tweaking applications necessary on newer C2D machines, but they're practically required for Core Duos. Especially if you want to use your machine on your lap.
As I mentioned yesterday, there is almost no downside to lowering processor voltage other than constant crashing. If your MacBook starts getting crashy, it's easy enough to crank the voltage back up a tad until it's stable again.
Although I still hope that Apple will include fan and temperature threshold control in a future version of the OS, you can rest assured that they'll never allow us to adjust the voltage. But that's what shareware developers are for.