While it isn't the first product turning a smartphone into a microscope, the Micro Phone Lens is surely among the simplest solutions around. The accessory is pretty much just a tiny lens that you stick onto your phone's camera, gaining extra magnification for taking a closer look at stuff like the tracks on a circuit board or the wings of a fly.
What you need to know about the Micro Phone Lens is that it adds 15X zoom to your camera's optical system. Sure, that isn't a whole lot when compared to a professional microscope, but seriously, you cannot really expect more from something that costs $15. Besides, a stand-alone microscope is quite bulky. The Micro Phone Lens, on the other hand, is made with portability in mind so it can be carried around anywhere. The accessory self-adheres to the camera you want to use it with (5MP or more with auto-focus recommended), meaning that special cases or attachments aren't needed. It is made of a material that is optically identical to glass and resistant to scratching.
Want to have one of these? Well, then Kicstarter is the place to go to. The project has raised sufficient funding already and anyone who pledged will get their Micro Phone Lens in October at the latest. And by the way, the guy who came up with the idea has a supercharged, 150X zoom lens in the works, so you may want to keep an eye out for that as well.