The 4-foot "classic beadboard" wall shelf is simple and cheap to make and it looks great for hanging pictures or decorations on the wall. The project requires only about $10 in materials, but it does required more advanced tools such as a chop saw, table saw, jig saw and router. Comparable shelves sell at between $60-$80.
I can build the project is about 1-2 hours, but it may take longer if you don't have all your tools set up. The plans are free on my woodworking blog at: http://www.jrlwoodworking.com/
You can also make the shelf in 2-foot and 3-foot lengths, simply shrink all the horizontal dimensions by one or two feet.