Modifying your photos with Instagram effects after you've shot a photo is nice, but what if you want to actually change the kind of image you capture? Photojojo's Lens Dial gives you three swappable iPhone lenses for wide angle, telephoto and fisheye.
The kit slips onto your iPhone, and you can easily turn the dial in order to swap lenses. The whole kit is $US250, which is pricey, and bulky, so you won't be able to slip your phone in and out of your pocket as easily. Also, it's a bad idea to keep those lenses exposed when you toss your phone into a purse, so I'm thinking this is mostly only for using when you're sure you're going out and taking photographs, rather than just having on you all the time.
If you've got spare lenses lying around from old cameras, you could probably manage to rig up your own system too. For example, you can use an old DVD lens to make a great macro lens.