Set MacBook Pro on energy saving mode I have a 2008 MacBook Pro (last one before the unibody ones) and have been monitoring a the laptop temperatures for almost a day. I used Temperature Monitor to graph the changes in temp and correlate them to my typical usage.  

Aside from obvious considerations (GPU getting HOT while playing 3D stuff), I noticed that when I unplugged the laptop from the AC adapter the temp dropped down by 15-30 °C.  

I think this may be related to some energy-saving mode of the CPU/GPU but I didn't find any setting or hack that allow me to turn it on on purpose. Is there a way to do so?  

PS: One may ask, why do you care about temperatures: well, first of all it's summer and this laptop is damn hot. On the other hand, I'd much prefer my laptop to use only as much resources it needs for common tasks and then turning it on when I need more computing power.
The temperature drop when unplugging the power can be attributed to Intel's SpeedStep technology, which will conserve energy while working on battery by dynamically decreasing/increasing the clock speed.

There is no way to control this from Mac OS X itself, but for your processor you could use the 3rd party software CoolBook. I have no experience with this software though, and I'd be cautious, there really is no good reason to go fiddling with these settings manuals.