xnu-speedstep-air is a kernel extension to keep my MacBook Air from overheating.
It's totally free and works with Mac OS X Lion. Compare to CoolBook which doesn't work with Lion and costs $10.
It's basically xnu-speedstep with bugfixes and preloaded voltage tables for the MacBook Air Rev. A.
I own a MacBook Air Rev. A, which is well-known for its thermal design problems.
Fortunately, if you reduce the voltage to the processor, the system can run much cooler without thermal protection events happening (like kernel_task emitting no-ops or core shutdown).
I used to use CoolBook for undervolting the system, but it costs $10. When I upgraded to Lion, CoolBook stopped working. Instead of downgrading to Snow Leopard, I decided to save my money by hacking this together.
You've saved my sanity and tought me a tiny bit about hacking OS X.
You should understand what this does: if you follow these instructions, you're undervolting your CPU to 892 - 940 mV. I use it on my MacBook Air from 2008 without a problem.
If you're using a different computer, you need to change or remove the PStateTable entry in Info.plist. I only tested it with the MacBook Air Rev. A., computers made after 2008 are very unlikely to work. Failure to be careful may cause hardware instability, crashes, or possibly damage.
Read the LICENSE file. This code is provided AS-IS and at your own risk.
Build the Source using Xcode. You may need to change your build product location (see below).
Run deploy.sh, type your password.
sudo kextload /System/Library/Extensions/IntelEnhancedSpeedStep.kext
Verify it worked:
sudo dmesg | grep IntelEnhancedSpeedStep
Xcode 4 and build products
In Preferences, select the Locations tab.
From the dropdown, select: Place build products in locations specified by targets
You'll now get the kext in build/.