Ground wires do just that, ground the electrical system to the neg terminal...as far as aftermarket "circle earth" stuff, not much to new cars, though older weather exposed vehicles can see some benefits to it...but no more than just replacing the ground wires with new ones. Improved grounds should only really be needed if you significantly upped the draw on the systems, such as big stereo, all sorts of extra lighting, an on board espresso maker etc.
Voltage stabilizers are caps...quick acting release of energy to make up for sudden dips, such as big stereo's give with bass hits. Their effectiveness is debatable, some saying they make all the difference, others saying they do next to nothing.