HYBSTER  You should be aware that the author of the program insists on a strict "one machine" license, you will only be able to use this with one computer. If you upgrade, you will have to pay again. Stupid policy? Up to you...

"The license is for one computer only."

-Magnus Lundholm  
(Version 2.9)

HYBSTER  You should be aware that the author of the program insists on a strict "one machine" license, you will only be able to use this with one computer. If you upgrade, you will have to pay again. Stupid policy? Up to you...

"The license is for one computer only."

-Magnus Lundholm  
(Version 2.9)

DRDUL  Great app! Installed this on my Mac mini (FWIW, my settings are 1670 MHz/0.95 V and 2004 MHz/1.00 V). Temperatures used to hit 70 degrees Celsius for no apparent reason, and now remain in the mid-40s.  
(Version 2.9)

JKAHAN  Excellent product. I just installed it on my Macbook Air and no longer have heat or core shutdowns. Search the posts on macrumors for "underclock" and there is a good thread comparing everyone's coolbook settings.

The GUI could use some work though....  
(Version 2.8)

THANAR  Looks like Coolbook throttling is broken under newest Mac OS X 10.5.2, although it used to work fine under 10.5.1.  
(Version 2.6)

JETFIREDX  Wouldn't install the driver on Leopard. I have the last version of the controller on the system from upgrading with migration assistant, but the last driver is not there since I formatted and started a clean install.  
(Version 2.5)

TOMASX  This app is well worth the tiny price. I have used it for almost a year now with a 17" Macbook pro (core 2 duo). The default settings left it running fairly hot, but after spending 20 minutes figuring out the minimum voltages needed per frequency I had some settings that have been totally stable.

When you're on battery you get much better performance and use less energy, by setting the voltage to the minimum but the frequency to the max that voltage can power (for me this was almost double the CPUs normal minimum).

and for desktop work you can use the max CPU speed with a big cut in power, meaning less heat and nose from fans.

In case it helps someone out, my settings turned out to be; 1837MHz-0.95V, 2004Mhz-0.9625V, 2171Mhz-1V, 2338Mhz-1.05V.  
(Version 2.3)

PAULVICTOLY  I purchased this today after I was informed by the developer that the latest version supported Santa Rosa based MBPs. My settings ended up being 0.85v for 1200mhz, 0.90v for 1600mhz, 0.925v for 2000mhz, and 1.0625v for 2400mhz. Not too bad!  
(Version 2.3)

JETFIREDX  Just registered today after reading the good reviews and wanting my MacBook to run cooler. Working well with my first level of undervolting. For $10 it seems to be well worth giving a shot. A few small suggestions: Would it be possible to have the ability to remove a single Frequency / Voltage pair from the lists? As it is, if you add the wrong values you have the clear the entire list and add them all again...a bit of a pain and I did this several times today messing with the settings. Second, tying in the ability to control the fan speed would be handy too. I always thought the fans kicked up too slowly on my MacBook, letting the heat build up more than it needed to. Having an option of setting minimum fan RPM per frequency would be nice because when I am running at full speed the fans could go faster before the temperature would normally tell them to kick up.   
(Version 2.3)

ERIC PETIT  Worth the $10

I have been using it for several months, my MBP CoreDuo 2Ghz is stable with the minimal voltage (0.95V) at 1000-1670Mhz, 0.9875V at 1837Mhz and 1.0625V at 2000Mhz - which makes it sensibly cooler.

Coolbook is also essential if, like me, you often use your MBP without the battery : Apple's defaults then limit the CPU at 1Ghz, while Coolbook will let you use any frequency.

One small feature request: I usually let it throttle within 1000-1670Mhz when I am on battery, except when I really need the longest battery life (like on the train) and keep the CPU at 1Ghz. It would be handy to be able to define presets so that I can easily switch between those two settings.  
(Version 2.2)