If you feel like having some fun and getting creative with your pictures, a must try is Shape Collage 2.1. It is a free collage maker that arranges your photos in all kinds of shapes in a matter of minutes. To achieve this, it uses a machine learning algorithm that impressively makes your pictures into well, um another picture.

http://www.apple.com/downloads/macosx/imaging_3d/shapecollage.html