If you want smoky looking lower lids or perfectly blended layering on your upper lid, a smudger is the tool for you.
The brush -- This brush isn't really a brush at all. It's a handle attached to a spongy or rubbery tip. The tip is effective at blunting eyeliner and shadow products so they go on diffusely. Smudge applicators are available in two sizes, a larger applicator for the upper eyelid and a slightly smaller applicator for the lower lid.
The technique -- Use a smudger the way you would a pencil eraser, with a gentle, even smoothing or rubbing motion. You can either apply liner or shadow using a smudger or use it to redistribute eye makeup applied with another tool.