You may already be aware of the olloclip lens for the iPhone. 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.
The first two are just for fun but the polarizer filter can be incredibly useful in cutting out all the glare and reflections from your pictures.
The whole set costs $35 and can be purchased from the link below. The design isn’t as elegant as olloclip’s but the addition of the polarizing filter itself makes it worth it, especially to those who take a lot of pictures from their iPhone.