Babies see the world usually up. We are told to place them on their tummies in order to develop their neck muscles and core strength in preparation for rolling and crawling but the majority of the time they are on their back looking heavenward.
Mobiles are great when it comes to helping your baby develop visual-motor skills. Because they move and are mobile, they encourage your baby to keep his focus on moving objects and control the direction of his gaze. You can easily make your own mobile or buy one to match in with your nursery. (high-contrast colors such as black, white, red, blue and yellow are easiest for babies to focus on).
One thing to remember is making sure you mobile and its parts are securely attached. Here are a few lovely examples to stimulate your little one.