When you own your Macbook Air for some time you can start hearing really noisy sounds from the inside out. This is due to the processor temperature and the fan installed on it (hello captain obvious! :). Problem is that you can't really control your fan by using system preferences or other embedded things in Mac OS X. And there you go to google and search for something like "Macbook Air noisy fan fix" etc..
Well! Your Mac decides when fan should speed up and slow down. But there is nice little free tool to use for such control.
This software allows you to control your fan RPM, even more you can set default values for cases when your Mac is charging or just on battery or full battery.
Nice little thing makes your life easier! ;-)