Floodlights can be used to illuminate your dormers. These types of lights are typically hung at the apex of the dormer overhang, facing down, to illuminate the face of the dormer. The light will spread down the house in an attractive way but draw the eye up to the dormer. This is known as down-lighting and can be good for security as well as aesthetics, since it will illuminate the area around your home.
Spotlights can be placed in the apex of the dormer facing down, but they can also be placed below the dormer facing up. Spotlights are adjustable, so you can move the focus of the light to illuminate your dormer in particular or to illuminate part of the walls of the house, as well. Because they are far from what they are illuminating, you can make the light intensely focused on the dormers or move them so that the lighting is more diffused, or you can use multiple lights on one window for a dramatic effect known as cross lighting.
Also known as well lighting, this kind of light consists of a small cylindrical housing with a light inside that is buried into the ground so that the top is flush with the ground. This kind of lighting is visually appealing because the source of the lighting is concealed. But you can't reposition the light the way you can for a spotlight. In-ground lighting is used for up-lighting on your home, so you can place one underneath each gable to light them from below.
If you find floodlights, spotlights and in-ground lighting are too dramatic, you can use other kinds of lighting for a more subtle effect. For instance, post lighting on a fairly tall post below the dormers can attract attention but won't provide dramatic illumination. Decorative lighting is normally for areas like porches, but an item like a hanging lantern in a dormer could give your home a custom look. There are also subtle lights, such as those designed to illuminate steps on a deck, that you could put into a dormer window overhang for a less intense look.