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GizmonClip-On Lenses gives the option to change the lense and also works with iPad.
It has a basic clip that slides on top of you iPhone aor iPad and then you get three interchangeable lenses that you can screw on top of the clip. There are fish-eye, 3 images mirage filter and circular polarizer lenses available by default.
The first two are just for fun but the polarizer filter can in incredibly useful in cutting out all the glare and reflections from your pictures.