PowerNow! is let's call it dynamic frequency scaling and power saving technology of AMD's processors used in laptops. The CPU's clock speed and VCore are automatically decreased when the computer is under low load or idle, to save battery power, reduce heat and noise. The lifetime of the CPU is also extended because of reduced electromigration, which varies exponentially with temperature.
The technology is a concept similar to Intel's SpeedStep technology. The adaptation of PowerNow! for AMD's desktop CPUs is called Cool'n'Quiet. Newer Opterons also use an adaptation of PowerNow! called Optimized Power Management.