Apple sets the fan speed to 1000 RPM by default. This choice results in a surface temperature well in excess of what’s comfortable on both the bottom and top of the “laptop”. In fact it’s so warm that Apple advises users to “not leave the bottom of your MacBook Pro in contact with your lap or any surface of your body for extended periods” in the owner’s manual.
By using this new tool and setting the minimum to a more reasonable 2000 or 2500 RPM you can have a still quiet laptop that can actually sit in your lap comfortably. The effect on battery life should be negligible compared to the big power consumers (e.g. the display backlight, the hard drive).
Best yet, it’s open source, not another $10 piece of “shareware”, so everyone can use and improve it.
Update: There’s an even better program for handling this called Fan Control that is also free software, and has a handy pane in System Preferences.