There are a few tutorials online for creating your own phone microscope using the lens of a laser pointer. (I tried this and it worked pretty well, but I never found a way to conveniently attach the lens to the camera.) Wired’s article Turn Your Cellphone Into a High-Powered Scientific Microscope has a good tutorial as well as background on the scientists who are using cellphones as biomedical devices.
Fortunately there are now multiple products for sale that make it easier to attach a lens to the phone.
Supereyes Smartphone Microscope (Not recommended)
I purchased the Supereyes Smartphone Microscope for about $7USD. The images below were taken with this lens. Unfortunately the top of the plastic lens is not protected and I scratched it after playing with it for less than an hour.
The images from the iPhone with attached lens look much better than my USB microscope. The photos below were taken in natural light.