The wide-angle macro and fisheye lenses from Photojojo come with a little self-adhesive magnetic ring that stick to your smart phone. The ring's removable. 
 The wide-angle gives a 0.67x angle of view, and will let you focus as close as 10mm. The fisheye will give a 180º view, and a 0.28x magnification. "The easiest way to make your best camera even better."