Ikea makes great, affordable picture ledge shelves that are perfect for this. Sadly, they don't ship and we don't have an Ikea nearby. So we made our own! We used cedar and stained it to match my living room furniture.
Each self is only 3 pieces of wood. The bottom which is 3 1/2 inches x the length you want your shelf (mine are about 4 feet and about 2 feet.)
The back piece (which is where you screw the shelves to the wall) is about 2 inches, and the front picture ledge is about 1 inch. You just glue all the pieces together and voila - a picture ledge shelf.
All you need to hang the shelves are screws and possibly anchors if you aren't hanging on studs. We used 3 screws/anchors for the large shelf, and 2 for the smaller ones.
Then I added framed pictures and some nic-nacs. I loved how it turned out!
I also added a collage frame in my living room. This is my All About Lola collage!