Gizmon Clip-On Lenses lets you add interchangeable lenses to your iPhone and iPad 

Gizmon Clip-On Lenses are similar but gives you the option to change the lens and also works with an iPad. 

You have a basic clip that slides on top of your iPhone or iPad and then you get three interchangeable lenses that you can screw on top of the clip. There are fish-eye, 3 image mirage filter and circular polarizer lenses available by default.