You can undervolt just about any CPU (AMD, Intel, Motorola, Samsung, etc). Most people with desktops want to overclock the CPU (make the CPU faster than what its rated for). Overclocking usually requires you overvolt your CPU. Of course, the amount of juice you use on a desktop doesn't matter because you are plugged into the wall.
With a laptop (using a battery), undervolting saves power. No performance changes what-so-ever. By using less power, it also runs cooler. Cooler CPU ----> Slower / no fans -----> Less power used / quieter laptop -----> Happier user!
The reason why CPU companies don't undervolt their chips to begin with is that not all chips can be undervolted to the same level, but ALL can run at the original level. In my case (Pentium M 1.6GHz; Inspiron 9300), my CPU originally was running at 1.356v (max; 1.6GHz) and 0.988v (min; 800MHz). I have mine down to 0.908v (max; 1.6GHz) and 0.700v (min; 800MHz). That's 33% less voltage (and power)! I dropped over 30F. I also reduced my power (this is while the CPU is running at 100%) by 11 watts! This is just by undervolting the CPU. No performance change, no cracking the case open.
The reason you need RightMark is because you can't set the voltage levels in the BIOS (like you can with most desktops). So, you need the software to change it for you when your computer starts up.
MobileMeter is also a GREAT utility to see how much power you save by undervolting. SuperPI is a great program to max your CPU out at 100%. FanGUI to manage fans.